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

Sphinx has been logging this warning 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'
```

There is no "processes" link in the client nav at http://docs.deis.io currently;
Sphinx just elides it in this case. It appears we forgot to rename the source
file from containers.rst to ps.rst, and doing so fixes the docset.