Update Counter.sol (#1917)

* Update Counter.sol

Added a comment in increment to make this more visible to not use SafeMath here.

* Update Counters.sol

contracts/drafts/Counters.sol

@@ -28,6 +28,7 @@ library Counters {
     }
 
     function increment(Counter storage counter) internal {
+        // The {SafeMath} overflow check can be skipped here, see the comment at the top
         counter._value += 1;
     }