apparmor: Fix doc comment for compute_fperms

When compute_fperms was moved to policy_compat and made static it
was renamed from aa_compute_fperms to just compute_fperms to help
indicate it is only available statically. Unfortunately the doc
comment did not also get updated to reflect the change.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>