Make KeyFileExt comment match reality

There is no file format version field as implemented.
Remove it from the format laid out in the comment, and change the
associated text to make more sense.

Change-Id: I3a7e4ca22bb4d556a091abc0e6e22d3409e419cd
Reviewed-on: http://gerrit.openafs.org/10020
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>