crypto/bn256/cloudflare: fix comments to describe the updated curve parameters (#19577)

* Removed comment section referring to Cloudflare's bn curve parameters

* Added comment to clarify the nature of the parameters

* Changed value of xi to i+9

crypto/bn256/cloudflare/gfp6.go

@@ -5,7 +5,7 @@ package bn256
 // http://eprint.iacr.org/2006/471.pdf.
 
 // gfP6 implements the field of size p⁶ as a cubic extension of gfP2 where τ³=ξ
-// and ξ=i+3.
+// and ξ=i+9.
 type gfP6 struct {
 	x, y, z gfP2 // value is xτ² + yτ + z
 }