releasenotes: fix documentation of similaritycheck()

The function document says that it returns true when the fragment can be merged,
but if you see the function just above it which is similar(), it writes already
exists thing if return value from similaritycheck() is False which is just
opposite of the doc. This patch fixes that.

Differential Revision: https://phab.mercurial-scm.org/D1119

hgext/releasenotes.py

@@ -216,7 +216,7 b' def similar(ui, existing, incoming_str):'
 
 def similaritycheck(incoming_str, existingnotes):
     """
-    Returns true when note fragment can be merged to existing notes.
+    Returns false when note fragment can be merged to existing notes.
     """
     import fuzzywuzzy.fuzz as fuzz
     merge = True