Correct Gerrit documentation link
Correct Gerrit documentation link

The documentation was moved, so update the link.

Also change the link to refer to the documentation to refer
to Gerrit 2.8 which is now the latest major version.

Change-Id: I07d55521b52806f09130bcb6673a354e241f2c92

benjiii/pygerrit,dpursehouse/pygerrit,markon/pygerrit2,gferon/pygerrit2,morucci/pygerrit,dpursehouse/pygerrit2,sonyxperiadev/pygerrit