docs: fix typo in memory.txt

The space between 7000 and 8000 is too wide by 1 character.
Also correct the range of vga-window example 0xa0000-0xbffff.

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Message-Id: <1458639954-9980-1-git-send-email-weijg.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>