math/big: fix incorrect comment variable reference

Fix comment as w&1 is the parity of 'x', not of 'n'.

Change-Id: Ia0e448f7e5896412ff9b164459ce15561ab624cc
GitHub-Last-Rev: 54ba08ab1055b5e6e506fc8ac06c2920ff095b6e
GitHub-Pull-Request: golang/go#29419
Reviewed-on: https://go-review.googlesource.com/c/155743Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/prime.go

@@ -51,7 +51,7 @@ func (x *Int) ProbablyPrime(n int) bool {
 	}
 
 	if w&1 == 0 {
-		return false // n is even
+		return false // x is even
 	}
 
 	const primesA = 3 * 5 * 7 * 11 * 13 * 17 * 19 * 23 * 37