qemu-nbd: Fix texi sentence capitalisation

Capitalise the first letter of sentences (and reword for grammar) the
options section of qemu-nbd.texi.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Message-Id: <1451979212-25479-4-git-send-email-sitsofe@yahoo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>