Fix the section name in CONTRIBUTING.rst

The name of the section "If you're a developer, start here"
has changed in the wiki, now is "If you're a developer".
This commit updates it to correctly refer to the
proper section.

Change-Id: I36c7e868f1c77d9efef0a0eadce9342212423c65
Merge "Fix the section name in CONTRIBUTING.rst"