Fix typo errors in devstack documentation

This submission fixes typo errors in following files
  1. contributing.rst
  2. exerciserc.rst
  3. faq.rst
  4. local.conf.rst
  5. openrc.rst

Change-Id: I2b2d61673c3c95f60c56978b5a81016603fef252
Merge "Fix typo errors in devstack documentation"
Merge branch 'master' of https://github.com/openstack-dev/devstack