crypto/internal/fips140/bigmod: fix extendedGCD comment

Change-Id: I6a6a6964642991dc46929bfc47e411bb7563e425
Reviewed-on: https://go-review.googlesource.com/c/go/+/716080
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: FiloSottile

src/crypto/internal/fips140/bigmod/nat.go

@@ -1088,7 +1088,7 @@ func (x *Nat) GCDVarTime(a, b *Nat) (*Nat, error) {
 	return x.set(u), nil
 }
 
-// extendedGCD computes u and A such that a = GCD(a, m) and u = A*a - B*m.
+// extendedGCD computes u and A such that u = GCD(a, m) = A*a - B*m.
 //
 // u will have the size of the larger of a and m, and A will have the size of m.
 //