Update contributing.rst instructions for local tests.

[mzyndul]

This PR has the objective of...

Changes

1. fix instruction for sphinx install
2. fix sample tox run ( replace docstest with docs )

Checklist

Checklist for pull requests including new code and/or changes to existing code...

• [ ] Code
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes documented in docs/changes.rst
• [x] Testing
• [x] Changes
  • [x] Documentation update
  • [x] Ready to review
  • [x] Ready to merge

[coveralls]

Coverage decreased (-1.3%) to 89.974% when pulling 758c9f3ff78be397492453836621e2d17ebe599b on mzyndul:553 into 3e7a8c611ec957e19526695c456987c9af54ffe0 on petl-developers:master.

[juarezr]

Great!