Markdown.pl: fix tab setting

A tab indents 8 spaces.  Period.
The coding style for this file is an indent of 4.
Correct the assume-tabs-are-broken-and-only-4-spaces problem.

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

Markdown.pl

@@ -3,8 +3,8 @@
 #
 # Markdown -- A text-to-HTML conversion tool for web writers
 #
-# Copyright (c) 2004 John Gruber
-# <http://daringfireball.net/projects/markdown/>
+# Copyright (C) 2004 John Gruber
+# Copyright (C) 2015 Kyle J. McKay
 #