Fix typo reported in #964592. Will backport to 2.3.

Doc/ref/ref2.tex

@@ -348,7 +348,7 @@ trailing underscore characters:
 
 \item[\code{__*}]
   Class-private names.  Names in this category, when used within the
-  context of a class definition, are re-written to use a mangled for
+  context of a class definition, are re-written to use a mangled form
   to help avoid name clashes between ``private'' attributes of base
   and derived classes.
   See section~\ref{atom-identifiers}, ``Identifiers (Names).''