improve command-line help for --enable_peer_stats

improve command-line help for --enable_peer_stats

The command-line help for several OpenAFS servers lists an inaccurate
description for the --enable_peer_stats option:

   "enable RX transport statistics"

Improve the help description to be more clear and consistent with the
description for --enable-process-stats.

Introduced by the following commits:
cd3492d volser: Convert command line parsing to cmd
a5effd9 viced: Use libcmd for command line options
461603e vlserver: Use libcmd for command line parsing
0b9986c ptserver: Use libcmd for command line parsing

Change-Id: Ibe23c61d4b838f3a3185390b18d25494fffde2ca
Reviewed-on: https://gerrit.openafs.org/14072
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>