doc: improve usage string in MyFirstContribution

doc: improve usage string in MyFirstContribution

We implement a command called git-psuh which accept arguments, so let's
show that it accepts arguments in the doc and the usage string.

While at it, we need to prepare "a NULL-terminated array of usage strings",
not just "a NULL-terminated usage string".

Helped-by: Emily Shaffer <emilyshaffer@google.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>