stringutil: edit comment to reflect actual data type name

In development the data type used to hold an email/name pair
was called a "mailmaptup" since it was implemented as a
namedtuple. The implementation has since been changed to use
an @attr.s decorated class named mailmapping. This commit
changes a comment to reflect this change.

Differential Revision: https://phab.mercurial-scm.org/D3004

mercurial/utils/stringutil.py

@@ -300,7 +300,7 b' def mapname(mailmap, author):'
     if not isauthorwellformed(author) or not mailmap:
         return author
 
-    # Turn the user name into a mailmaptup
+    # Turn the user name into a mailmapping
     commit = mailmapping(name=person(author), email=email(author))
 
     try: