[rbridge] Making VLAN mapping not illegal -- minor wording change

[Radia Perlman]

Section 4.1.2 says "When a TRILL frame passes through a
transit RBridge, the Inner.VLAN MUST NOT be changed."

If we were to do VLAN mapping, (draft to come...) we would want to allow 
an RBridge
to change the inner VLAN tag. So the wording perhaps should be:

"....the Inner.VLAN MUST NOT be changed, except when VLAN mapping is
being intentionally performed."