BGP: fix dissection of BGP community tag

The offset used for BGP community tag dissection is a wrong one.

Bug: 10746
Change-Id: I1d1d443568bb97a0b3b95a312762ac0a3102326a
Reviewed-on: https://code.wireshark.org/review/5562
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>