qemu-doc.texi: remove trailing whitespace

Remove trailing whitespace in qemu-doc.texi, as it causes
reproducibility issues depending on the echo implementation
used by the Makefile.

Reported-By: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>