doc: escape left brace in texi2pod.pl regex

Unescaped literal left braces are deprecated and a warning was added in
Perl 5.22.

doc/texi2pod.pl

@@ -380,7 +380,7 @@ sub postprocess
     # @* is also impossible in .pod; we discard it and any newline that
     # follows it.  Similarly, our macro @gol must be discarded.
 
-    s/\@anchor{(?:[^\}]*)\}//g;
+    s/\@anchor\{(?:[^\}]*)\}//g;
     s/\(?\@xref\{(?:[^\}]*)\}(?:[^.<]|(?:<[^<>]*>))*\.\)?//g;
     s/\s+\(\@pxref\{(?:[^\}]*)\}\)//g;
     s/;\s+\@pxref\{(?:[^\}]*)\}//g;