spelling: github (#900)

Signed-off-by: Josh Soref <[email protected]>

Author: jsoref

CONTRIBUTING.md

@@ -14,7 +14,7 @@ By participating in this project you agree to abide by its terms.
 
 Before you create a new Issue:
 
-- Check the [Issues](https://github.com/octokit/graphql-schema/issues) on Github to ensure one doesn't already exist.
+- Check the [Issues](https://github.com/octokit/graphql-schema/issues) on GitHub to ensure one doesn't already exist.
 - Clearly describe the issue, including the steps to reproduce the issue.
 - Please include links to the corresponding github documentation.