oeqa context.py: fix --target-ip comment to include ssh port number

Providing ssh port number is supported too with
"--target-ip 192.168.0.10:22".

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 637919b9)
Signed-off-by: Steve Sakoman <steve@sakoman.com>