Merge pull request sendgrid#351 from xjlim/fix-broken-link

Update CONTRIBUTING.md

Author: thinkingserious

CONTRIBUTING.md

@@ -61,7 +61,7 @@ We welcome direct contributions to the sendgrid-python code base. Thank you!
 ### Development Environment ###
 
 #### Using Docker ####
-You can use our Docker image to avoid setting up the development environment yourself.  See [USAGE.md](https://github.com/sendgrid/sendgrid-python/docker/USAGE.md).
+You can use our Docker image to avoid setting up the development environment yourself.  See [USAGE.md](https://github.com/sendgrid/sendgrid-python/blob/master/docker/USAGE.md).
 
 #### Install and Run Locally ####