fix(docs): rename client/containers.rst to ps.rst

Sphinx has been logging this error since #813 was merged:
```console
docs matt$ make server
sphinx-build -b dirhtml -d _build/doctrees   . _build/dirhtml
...
/Users/matt/Projects/deis/docs/client/index.rst:12: WARNING: toctree contains reference to nonexisting document u'client/ps'
```

It appears we forgot to rename the source file from containers.rst
to ps.rst, and doing so fixes the docset.
Merge pull request #841 from deis/fix-client-ps-docs

fix(docs): rename client/containers.rst to ps.rst