Merge pull request sendgrid#370 from ciceropablo/patch-1

Update CONTRIBUTING.md

Author: thinkingserious

CONTRIBUTING.md

@@ -131,7 +131,7 @@ For Python 2.7.* and up:
 
 All PRs require passing tests before the PR will be reviewed.
 
-#### Prequisites: ####
+#### Prerequisites: ####
 
 The above local "Initial setup" is complete