am: align placeholder for --whitespace option with apply

am: align placeholder for --whitespace option with apply

`git am` passes the value given to its `--whitespace` option through
to the underlying `git apply`, and the value is called <action> over
there.  Fix the documentation for the command that calls the value
<option> to say <action> instead.

Note that the option help given by `git am -h` already calls the
value <action>, so there is no need to make a matching change there.

Signed-off-by: Junio C Hamano <gitster@pobox.com>