Apply correct language identifiers to the code blocks in the document…

…ation on how to work with schemas (apache#6089)

### Motivation

When I read the documentation on how to work with schemas I saw that some code blocks are not highlighted, because of incorrect language identifiers.

### Modifications

Correct language identifiers were applied.

site2/docs/schema-understand.md

@@ -110,7 +110,7 @@ Possible properties might be the Git hash associated with the schema, an environ
 
 This is the `SchemaInfo` of a string.
 
-```text
+```json
 {
     "name": "test-string-schema",
     "type": "STRING",
@@ -171,14 +171,14 @@ This example demonstrates how to use a string schema.
 
 1. Create a producer with a string schema and send messages.
 
-    ```text
+    ```java
     Producer<String> producer = client.newProducer(Schema.STRING).create();
     producer.newMessage().value("Hello Pulsar!").send();
     ```
 
 2. Create a consumer with a string schema and receive messages.  
 
-    ```text
+    ```java
     Consumer<String> consumer = client.newConsumer(Schema.STRING).create();
     consumer.receive();
     ```
@@ -305,7 +305,7 @@ Pulsar gets the schema definition from the predefined `struct` using an Avro lib
 
 1. Create the _User_ class to define the messages sent to Pulsar topics.
 
-    ```text
+    ```java
     public class User {
         String name;
         int age;
@@ -314,14 +314,14 @@ Pulsar gets the schema definition from the predefined `struct` using an Avro lib
 
 2. Create a producer with a `struct` schema and send messages.
 
-    ```text
+    ```java
     Producer<User> producer = client.newProducer(Schema.AVRO(User.class)).create();
     producer.newMessage().value(User.builder().userName("pulsar-user").userId(1L).build()).send();
     ```
 
 3. Create a consumer with a `struct` schema and receive messages
 
-    ```text
+    ```java
     Consumer<User> consumer = client.newConsumer(Schema.AVRO(User.class)).create();
     User user = consumer.receive();
     ```
@@ -336,7 +336,7 @@ You can define the `struct` schema using the `GenericSchemaBuilder`, generate a
 
 1. Use `RecordSchemaBuilder` to build a schema.
 
-    ```text
+    ```java
     RecordSchemaBuilder recordSchemaBuilder = SchemaBuilder.record("schemaName");
     recordSchemaBuilder.field("intField").type(SchemaType.INT32);
     SchemaInfo schemaInfo = recordSchemaBuilder.build(SchemaType.AVRO);
@@ -346,7 +346,7 @@ You can define the `struct` schema using the `GenericSchemaBuilder`, generate a
 
 2. Use `RecordBuilder` to build the struct records.
 
-    ```text
+    ```java
     producer.newMessage().value(schema.newRecordBuilder()
                 .set("intField", 32)
                 .build()).send();
@@ -377,7 +377,7 @@ Suppose that:
    
 In this case, you can use `AUTO_PRODUCE` to verify whether the bytes produced by _K_ can be sent to _P_ or not.
 
-```text
+```java
 Produce<byte[]> pulsarProducer = client.newProducer(Schema.AUTO_PRODUCE())
     …
     .create();
@@ -405,14 +405,13 @@ Suppose that:
    
 In this case, you can use `AUTO_CONSUME` to verify whether the bytes produced by _P_ can be sent to MySQL or not.
 
-```text
+```java
 Consumer<GenericRecord> pulsarConsumer = client.newConsumer(Schema.AUTO_CONSUME())
     …
     .subscribe();
 
 Message<GenericRecord> msg = consumer.receive() ; 
 GenericRecord record = msg.getValue();
-…
 ```
 
 ## Schema version
@@ -431,7 +430,7 @@ The following example illustrates how the schema version works.
 
 Suppose that a Pulsar [Java client](client-libraries-java.md) created using the code below attempts to connect to Pulsar and begins to send messages:
 
-```text
+```java
 PulsarClient client = PulsarClient.builder()
         .serviceUrl("pulsar://localhost:6650")
         .build();