Fix pull request branch name in CONTRIBUTING
Fix pull request branch name in CONTRIBUTING

Update CONTRIBUTING to advise opening pull requests against the
`develop` branch, not `master`. This advice contradicts all other
documentation (including earlier in the same file) and is outdated.

sendgrid/docs,sendgrid/docs,Whatthefoxsays/docs,sendgrid/docs,Whatthefoxsays/docs,Whatthefoxsays/docs