doc: fix careless document miss for term 'Close changeset' in glossary

there is no term 'Changeset, close' in glossary.

'Close changeset' seems to have to be linked not to 'Branch, closed',
but to 'Head, closed branch', because only the latter explains about
"the changeset that marks a head as no longer interesting".

mercurial/help/glossary.txt

@@ -135,7 +135,7 b' Child changeset'
     See 'Changeset, child'.
 
 Close changeset
-    See 'Changeset, close'.
+    See 'Head, closed branch'
 
 Closed branch
     See 'Branch, closed'.