doc/memory.txt: correct a logic error

In the regions overlap example, region B has a higher priority thus
should has a larger priority number than C.

Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-Id: <1456476051-15121-1-git-send-email-zxq_yx_007@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>