Fix incorrect import for RemovalPolicy

In the example showing how to specify a RemovalPolicy the incorrect import was being specified:

```java
import software.amazon.awscdk.core.RemovalPolicy;
```

instead of 

```java
import software.amazon.awscdk.services.s3.BucketEncryption;
```

doc_source/hello_world.md

@@ -432,7 +432,7 @@ bucket = s3.Bucket(self, "MyFirstBucket",
 Update `src/main/java/com/myorg/HelloCdkStack.java`, adding the new import and updating the bucket definition in the appropriate places\.
 
 ```
-import software.amazon.awscdk.services.s3.BucketEncryption;
+import software.amazon.awscdk.core.RemovalPolicy;
 ```
 
 ```
@@ -585,4 +585,4 @@ Where do you go now that you've dipped your toes in the AWS CDK?
 + Dig deeper into concepts like [Environments](environments.md), [Assets](assets.md), [Permissions](permissions.md), [Runtime context](context.md), [Parameters](parameters.md), and [Escape hatches](cfn_layer.md)\.
 + Explore [Examples](https://github.com/aws-samples/aws-cdk-examples) of using the AWS CDK\.
 
-The AWS CDK is an open\-source project\. Want to [contribute](https://github.com/aws/aws-cdk)?
+The AWS CDK is an open\-source project\. Want to [contribute](https://github.com/aws/aws-cdk)?