doc: remove "feature branch" jargon

In the context of the CONTRIBUTING.md document, there is no advantage to
describing the branch as a "feature branch" and the term may be
confusing. Just refer to it as a branch.

Context for the curious: The phrase "feature branch" in CONTIRUBTING.md
confused someone I was assisting today. It occured to me that the word
"feature" doesn't add anything and can be confusing. ("I'm doing a bug
fix so I don't need to create a feature branch, right?")

PR-URL: https://github.com/nodejs/node/pull/8194


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>