Changes reference to BSD license to MIT (facebookarchive#2130)

Summary:
**Summary**
Changes reference to the BSD license to MIT in the contributing guidelines. Fixes facebookarchive#2129

**Test Plan**
Proofreading
Pull Request resolved: facebookarchive#2130

Reviewed By: niveditc

Differential Revision: D16286748

Pulled By: claudiopro

fbshipit-source-id: 3700f15d32fd2b1527135e8f432600009051208e

Author: claudiopro

CONTRIBUTING.md

@@ -73,4 +73,4 @@ Our philosophy regarding API changes is as follows:
 
 ## License
 By contributing to Draft.js, you agree that your contributions will be licensed
-under its BSD license.
+under its MIT license.