Avoid referencing code from master branch
Avoid referencing code from master branch

I clicked on this link while reading the plugin development section in
the effective neutron guide.  I wasn't sure it linked me to the right
thing because it currently links to a blank line.  I figured that it
was just a line off but I wasn't sure because the link is to the
master branch, which can change.

This patch changes master to the short commit id of the commit that
added this link in the first place.  This way, it'll always link to
what it originally linked to.  If contents move around on master by
adding code to the file or refactoring or whatever, it might still
cause some confusion but at least we'll know what it was originally
intended to point to without doubt.

Change-Id: I13a10e10a31b88cb6f030ca644ca20e0bf14012c

openstack/neutron,bigswitch/neutron,eayunstack/neutron,huntxu/neutron,noironetworks/neutron,bigswitch/neutron,mahak/neutron,igor-toga/local-snat,sebrandon1/neutron,cloudbase/neutron,cloudbase/neutron,sebrandon1/neutron,huntxu/neutron,noironetworks/neutron,eayunstack/neutron,igor-toga/local-snat,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron