Rename dfprime2 => dfprm2
Rename dfprime2 => dfprm2

Since ` Prime ` is defined using df-prm, labels should use
"prm" (not "prime") when referring to it.
This rename eliminates the inconsistency.

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

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