Markdown.pl: use passed in prefix for GenerateStyleSheet

Although GenerateStyleSheet did, in fact, accept a prefix argument
(properly defaulting if omitted), it was not using the passed in
prefix.

Correct that so the style sheet can be generated using any desired
prefix, but most helpfully using the `style_prefix` as passed in
to the Markdown function.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -556,7 +556,7 @@ sub GenerateStyleSheet {
     my $prefix = shift;
     defined($prefix) or $prefix = $g_style_prefix;
     my $stylesheet = $g_style_sheet;
-    $stylesheet =~ s/%\(base\)/$g_style_prefix/g;
+    $stylesheet =~ s/%\(base\)/$prefix/g;
     return $stylesheet;
 }