Markdown.pl: avoid use of '

Although ' is specified as part of the XML standard,
some older end user clients may not actually recognize it.

Use ' instead of ' to avoid any difficulty.

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

Markdown.pl

@@ -2042,7 +2042,7 @@ sub _HTMLEncode {
 
     # Remaining entities now
     $text =~ s/\042/"/g;
-    $text =~ s/\047/'/g;
+    $text =~ s/\047/'/g; # Some older browsers do not grok '
     $text =~ s/</&lt;/g;
     $text =~ s/>/&gt;/g;