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: I16fc339292ac5766530f003770bee133170bed67

CONTRIBUTING.rst

@@ -1,8 +1,7 @@
 If you would like to contribute to the development of OpenStack,
-you must follow the steps in the "If you're a developer, start here"
-section of this page:
+you must follow the steps documented at:
 
-   http://wiki.openstack.org/HowToContribute
+   http://wiki.openstack.org/HowToContribute#If_you.27re_a_developer
 
 Once those steps have been completed, changes to OpenStack
 should be submitted for review via the Gerrit tool, following