tests/fuzzers/bls12381: fix error message in fuzzCrossG2Add (#31113)

Fixes a typo in the error message within the `fuzzCrossG2Add`
function. The panic message incorrectly references "G1 point addition
mismatch" when it should be "G2 point addition mismatch," as the
function deals with G2 points.

This doesn't affect functionality but could cause confusion during
debugging. I've updated the message to reflect the correct curve.

tests/fuzzers/bls12381/bls12381_fuzz.go

@@ -168,7 +168,7 @@ func fuzzCrossG2Add(data []byte) int {
 
 	bl3 := blst.P2AffinesAdd([]*blst.P2Affine{bl1, bl2})
 	if !(bytes.Equal(gp.Marshal(), bl3.Serialize())) {
-		panic("G1 point addition mismatch blst / geth ")
+		panic("G2 point addition mismatch blst / geth ")
 	}
 
 	return 1