Documentation: fix warning in cat-texi.perl

Newer versions of Perl produce the warning "Unescaped left brace in
regex is deprecated, passed through in regex" when an unescaped left
brace occurs in a regex.  Escape the brace to avoid this warning.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/cat-texi.perl

@@ -11,7 +11,7 @@ while (<STDIN>) {
 	if (s/^\@top (.*)/\@node $1,,,Top/) {
 		push @menu, $1;
 	}
-	s/\(\@pxref{\[(URLS|REMOTES)\]}\)//;
+	s/\(\@pxref\{\[(URLS|REMOTES)\]}\)//;
 	s/\@anchor\{[^{}]*\}//g;
 	print TMP;
 }