message: Update longest-nice-substring.cpp
diff: @@ -16,7 +16,7 @@ public:
  if (prev == -1 && i == size(s)) {
  return s;
  }
  - const auto tmp = longestNiceSubstring(s.substr(prev + 1, i - prev - 1));
  + auto tmp = longestNiceSubstring(s.substr(prev + 1, i - prev - 1));
  if (size(tmp) > size(result)) {
  result = move(tmp);
  }
  