Fix inconsistency in hashtags event name and hook documentation

In the stream events documentation the hashtags changed event is named
"hashtags-changed", but the code was using "hashtags-edited". Update
the code to match the documentation.

In the hooks documentation the example was showing "hashtags-edited".
Correct it.

Change-Id: I84fc51b260c9a3d054ce3f23737e3098bd75a021