[hotfix][javadoc] Fix typo in StreamElement javadoc

This closes apache#5152.

flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/streamstatus/StreamStatus.java

@@ -63,7 +63,7 @@
  * active. However, for watermarks, since there may be watermark generators that might produce watermarks
  * anywhere in the middle of topologies regardless of whether there are input data at the operator, the current
  * status of the task must be checked before forwarding watermarks emitted from
- * an operator. It the status is actually idle, the watermark must be blocked.
+ * an operator. If the status is actually idle, the watermark must be blocked.
  *
  * <li>For downstream tasks with multiple input streams, the watermarks of input streams that are temporarily idle,
  * or has resumed to be active but its watermark is behind the overall min watermark of the operator, should not