BGP : Simple typo fix for BGP Attribute

Change-Id: I793eed5c7d63c82e13847c8a6c8fd6def3025b8c
Reviewed-on: https://code.wireshark.org/review/6158
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Hadriel Kaplan <hadrielk@yahoo.com>