doc: use "tag" revset predicate instead of "tagged" for example in help

"tag" predicate is officially described in help, but "tagged" is not,
even though the latter works as same as the former.

mercurial/help/revsets.txt

@@ -118,4 +118,4 b' Some sample queries:'
 - Changesets mentioning "bug" or "issue" that are not in a tagged
   release::
 
-    hg log -r "(keyword(bug) or keyword(issue)) and not ancestors(tagged())"
+    hg log -r "(keyword(bug) or keyword(issue)) and not ancestors(tag())"