stream_settings: Modify the message of the "Announce stream" hint tooltip.

Follow-up to commit bbda7a5bb0.

The "Announce stream" hint tooltip earlier read "Organization
administrators can change this in in the organization settings."
It wasn't obvious that "this" refers to the stream the notification
will go to, so the tootltip message has been modified to reflect this.

static/templates/announce_stream_docs.hbs

@@ -4,6 +4,6 @@
     {{#tr}}
     <p>Stream will be announced in <b>#{notifications_stream}</b>.</p>
     {{/tr}}
-    <p>{{t 'Organization administrators can change this in the organization settings.' }}</p>
+    <p>{{t 'Organization administrators can change the announcement stream in the organization settings.' }}</p>
 
 </div>