Remove -a flag from reference doc

[josesa]

File: compose/reference/rm.md

Documentation refers to an optional flag -a.

Options:
-a, --all Also remove one-off containers created by
docker-compose run

But when running latest (docker-compose version 1.11.2, build dfed245) docker-compose rm with the mentioned flag a warning is displayed:

WARNING: --all flag is obsolete. This is now the default behavior of `docker-compose rm`

Documentation should be updated to reflect the new behaviour.

[shubheksha]

I'll take this one up!

[mdlinville]

No action for two days, so this is up for grabs!

[shubheksha]

@mstanleyjones, a PR is open for this issue.

[mdlinville]

Whoops, my bad!