Enable TLS in link to chris.beams.io

Author: JeremyRand

CONTRIBUTING.md

@@ -62,7 +62,7 @@ Commit messages should be verbose by default consisting of a short subject line
 paragraph(s), unless the title alone is self-explanatory (like "Corrected typo
 in init.cpp") in which case a single title line is sufficient. Commit messages should be
 helpful to people reading your code in the future, so explain the reasoning for
-your decisions. Further explanation [here](http://chris.beams.io/posts/git-commit/).
+your decisions. Further explanation [here](https://chris.beams.io/posts/git-commit/).
 
 If a particular commit references another issue, please add the reference. For
 example: `refs #1234` or `fixes #4321`. Using the `fixes` or `closes` keywords