cli: Improve `flynn release` help text
cli: Improve `flynn release` help text

This format matches that of `flynn cluster` and is compatible with
the HTML doc generator.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

flynn/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,flynn/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn