qemu-options: mention C-a h in the -nographic doc

qemu-options: mention C-a h in the -nographic doc

Otherwise, a new user will be wondering how to switch between the
console and monitor.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>