testing/quick: fix comment

There is no top-level Values function.

Change-Id: I3ea2eea0b5f77f3e1a3f75d1a6472507ef2888bb
Reviewed-on: https://go-review.googlesource.com/8196Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/quick/quick.go

@@ -161,7 +161,7 @@ type Config struct {
 	Rand *rand.Rand
 	// If non-nil, the Values function generates a slice of arbitrary
 	// reflect.Values that are congruent with the arguments to the function
-	// being tested. Otherwise, the top-level Values function is used
+	// being tested. Otherwise, the top-level Value function is used
 	// to generate them.
 	Values func([]reflect.Value, *rand.Rand)
 }