Use imperative phrasing for command summaries

This makes everything consistent with the version command's:

  Print the runtime version and exit.

And follows the practice recommended by Python's PEP 257 [1]:

  The docstring is a phrase ending in a period. It prescribes the
  function or method's effect as a command ("Do this", "Return that"),
  not as a description; e.g. don't write "Returns the pathname ...".

[1]: https://www.python.org/dev/peps/pep-0257/#one-line-docstrings

Signed-off-by: W. Trevor King <wking@tremily.us>
Reviewed-by: Mike Brown <brownwm@us.ibm.com>

runtime.md

@@ -43,7 +43,7 @@ $ echo $?
 
 ### start
 
-Starts a container from a bundle directory.
+Start a container from a bundle directory.
 It operates by default on the `config.json` and `runtime.json` in the current directory.
 
 * *Options*