procfs.4: Fix the description of credential info in the "status" file

The 11th and 12th columns are the effective and real UIDs, respectively.
The 13th column is a group list whose first member is the effective
GID.

Reviewed by: arrowd
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45700

(cherry picked from commit b75d14d6005b36fe8ea90cadc027272eb06d1bc5)