Update README

Day3/README.md

@@ -111,6 +111,9 @@ public class HammingDistance {
 
  int hammingDistance(string first_word, string second_word) {
      int count = 0;
+     if (first_word.size() != second_word.size()) {
+        return -1;
+     }
      for (int i = 0; i < first_word.size(); i++) {
          if (first_word[i] != second_word[i]) {
              count++;