Fix permission on file

Removed execute permissions on prometheus_cfg.erl as it isn't needed and
raised the question of why it was that way.

Change-Id: I8a3e788f65c084f1007c53c22ed55a1336dd4ffb
Reviewed-on: http://review.couchbase.org/c/ns_server/+/158769
Tested-by: Steve Watanabe <[email protected]>
Reviewed-by: Timofey Barmin <[email protected]>
Well-Formed: Build Bot <[email protected]>

Author: stevewatanabe