fix outdated link.

Author: lucky521

README.markdown

@@ -517,7 +517,7 @@ Here's some tips if you've never used VIM before:
 * Certain commands are prefixed with a `<Leader>` key, which by default maps to `\`.
   Spf13-vim uses `let mapleader = ","` to change this to `,` which is in a consistent and
   convenient location.
-* Keyboard [cheat sheet](https://walking-without-crutches.heroku.com/image/images/vi-vim-cheat-sheet.png).
+* Keyboard [cheat sheet](http://www.viemu.com/vi-vim-cheat-sheet.gif).
 
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