Rename "df-ring" to "df-rng" for consistency
Rename "df-ring" to "df-rng" for consistency

Labels that refer to rings consistently refer to "rng",
so change the name of the definition to match.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm