ipv4: fix typo in package documentation

Change-Id: I73a93368cdff6cb9938dbe6b1dda05133a1043f5
Reviewed-on: https://go-review.googlesource.com/12629
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/doc.go

@@ -114,7 +114,7 @@
 //			// error handling
 //		}
 //		if cm.Dst.IsMulticast() {
-//			if cm.Dst.Equal(group)
+//			if cm.Dst.Equal(group) {
 //				// joined group, do something
 //			} else {
 //				// unknown group, discard