CONTRIBUTING.md: Capitalize "GitHub" the way GitHub does
CONTRIBUTING.md: Capitalize "GitHub" the way GitHub does

Fixes #200

MOE_MIGRATED_REVID=170552533

cushon/google-java-format,cushon/google-java-format,sormuras/google-java-format,cushon/google-java-format,sormuras/google-java-format,sormuras/google-java-format