docker help -> grammatical correction (pos. adj. apost.)

The help text for the `cp` command, "Copy files/folders from the containers filesystem",
exposed via `docker help`, needs amending (pos. det. - apostrophe).

Possible options:

 - [ ] "the containers' fi.."; or,
 - [x] "a container's filesystem".

This PR amends the sentence, going with the latter.

E.g. (ref.): http://en.wikipedia.org/wiki/Possessive_determiner

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)