Fix GSSAPI authind attribute name in docs

The correct attribute name is "auth-indicators".

(cherry picked from commit e3f666fe61e94e96d3c4d449e2fa8bd69cd2ecec)

https://github.com/krb5/krb5/commit/7e9b8c09f0a032642e27959e1d70cc62cc3634bc
Author: Matt Rogers <mrogers@redhat.com>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 7e9b8c09f0a032642e27959e1d70cc62cc3634bc
Branch: krb5-1.15
 doc/admin/auth_indicator.rst |    2 +-
 doc/appdev/gssapi.rst        |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)