net: tweak comment on ExampleCIDRMask

CIDRMask just returns a mask which corresponds to an address
prefix in CIDR nonation. A subnet for an IPv6 mask sounds a bit
confusing.

Change-Id: Ic7859ce992bc2de4043d3b25caf9a1051d118b0e
Reviewed-on: https://go-review.googlesource.com/33262Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/example_test.go

@@ -37,11 +37,13 @@ func ExampleListener() {
 }
 
 func ExampleCIDRMask() {
-	// This mask corresponds to a /31 subnet
+	// This mask corresponds to a /31 subnet for IPv4.
 	fmt.Println(net.CIDRMask(31, 32))
 
-	// This mask corresponds to a /64 subnet for a IPv6 mask
+	// This mask corresponds to a /64 subnet for IPv6.
 	fmt.Println(net.CIDRMask(64, 128))
-	// Output: fffffffe
+
+	// Output:
+	// fffffffe
 	// ffffffffffffffff0000000000000000
 }