Fix up $N vs \N in man page

The VCL man page documented the capturing parentheses as using $N
rather than \N which is actually used.

Fixes #359


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3361 d4fa192b-c00b-0410-8231-f00ffab90ce4

man/vcl.7so

@@ -198,11 +198,11 @@ replaced with
 .Fa sub .
 Within
 .Fa sub ,
-.Va $0
+.Va \\0
 (which can also be spelled
 .Va & )
 is replaced with the entire matched string, and
-.Va $n
+.Va \\n
 is replaced with the contents of subgroup
 .Ar n
 in the matched string.