Merge pull request #1891 from mari-linhares/fixing_contributing_typo

Fixing small typo in CONTRIBUTING.md

CONTRIBUTING.md

 # Contributing guidelines
 
 If you have created a model and would like to publish it here, please send us a
-pull request. For those just getting started with pull reuests, GitHub has a
+pull request. For those just getting started with pull requests, GitHub has a
 [howto](https://help.github.com/articles/using-pull-requests/).
 
 The code for any model in this repository is licensed under the Apache License