Fix the section name in CONTRIBUTING.rst
Fix the section name in CONTRIBUTING.rst

The name of the section "If you're 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: Ifb6d65eade55cdb7fbe723e75d12a44b9dd954b4

tcpcloud/python-heatclient,tcpcloud/python-heatclient,ecerulm/python-heatclient,openstack/python-heatclient,rdo-management/python-heatclient,rdo-management/python-heatclient,ecerulm/python-heatclient