Add weight == 0 condition to giveRightToVote function

This will make it so votes can't be deleted accidentally by giving someone with weight > 1 the right to vote again

docs/solidity-by-example.rst

@@ -94,7 +94,7 @@ of votes.
             // called incorrectly. But watch out, this
             // will currently also consume all provided gas
             // (this is planned to change in the future).
-            require((msg.sender == chairperson) && !voters[voter].voted);
+            require((msg.sender == chairperson) && !voters[voter].voted && (voters[voter].weight == 0));
             voters[voter].weight = 1;
         }