coreos-install: Cleanup usage message
coreos-install: Cleanup usage message

Fix the invocation and for consistency, add a period to the end
of some usage lines.

Signed-off-by: Geoff Levand <c3918f39022407dee6a0056ceca9b1aa80e8e4ce@infradead.org>

coreos/init,coreos/init,coreos/init