MIPS: qemu-mips: fix a typo in README

MIPS: qemu-mips: fix a typo in README

The 'Limitations & comments' section refers to the
'-m mips' switch which is not valid. The '-m' switch
can be used for setting the virtual RAM size:

    $qemu-system-mips --help | grep '^-m '
    -m megs         set virtual RAM size to megs MB [default=128]
    $

The correct switch for specifying the machine type is '-M'.
Fix the text to refer to that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Vlad Lungu <vlad.lungu@windriver.com>