Fixes grammatical errors in HACKING.rst

There are a few lines in HACKING.rst that were incorrect
grammatically. This fix changes those.

Change-Id: I8173dae3cf3a3e78a504b4edd4a7c1ff5b7fb9a4
Closes-Bug: #1389603
Merge "Fixes grammatical errors in HACKING.rst"
Merge branch 'master' of https://github.com/openstack/tempest