FAQ.md: Correct claims about MPLS.

An examination of the source code and the tests shows that the FAQ claimed
more for version 2.3 than was really in there.

Reported-by: null pointer <null.pointer.boom@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>