Add cross-reference from df-nf to nf2.
Add cross-reference from df-nf to nf2.

Some people may find the nf2 formulation easier to understand,
so provide a hyperlink from df-nf to nf2.

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

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