Update readthedocs link

We just got an email from readthedocs saying they are switching to readthedocs.io, for security reasons.
md384 · Apr 28, 2016 · 0b19541 · 0b19541
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commit 0b19541
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -12,7 +12,7 @@ Getting started, building, and testing
 
 If you haven't already, take a look at the project's
 [README.md file](README.md)
-and the [Mypy documentation](http://mypy.readthedocs.org/en/latest/),
+and the [Mypy documentation](http://mypy.readthedocs.io/en/latest/),
 and try adding type annotations to your file and type-checking it with Mypy.