Fixed admin-api-non-persistent-topics documentation (apache#3781)

Fixes apache#3780


### Motivation

Update the documentation to more accurately reflect the condition of the queue when the availablePermits value is negative. The current documentation indicates that condition means the consumer is ready to receive more messages, when, in fact, it is NOT.

### Modifications

Updated the http://pulsar.apache.org/docs/en/admin-api-non-persistent-topics.html#get-stats page

site2/docs/admin-api-non-persistent-topics.md

@@ -52,7 +52,7 @@ It shows current statistics of a given non-partitioned topic.
 
   -   **consumerName**: Internal identifier for this consumer, generated by the client library
 
-  -   **availablePermits**: The number of messages this consumer has space for in the client library's listen queue. A value of 0 means the client library's queue is full and receive() isn't being called. A nonzero value means this consumer is ready to be dispatched messages.
+  -   **availablePermits**: The number of messages this consumer has space for in the client library's listen queue. A value less than 1 means the client library's queue is full and receive() isn't being called. A non-negative value means this consumer is ready to be dispatched messages.
 
   -   **replication**: This section gives the stats for cross-colo replication of this topic