message: Update sort-the-matrix-diagonally.cpp
diff: @@ -10,7 +10,7 @@ public:
  lookup[i - j].emplace_back(mat[i][j]);
  }
  }
  - for (auto& [k, v] : lookup) {
  + for (auto& [_, v] : lookup) {
  sort(v.begin(), v.end());
  }
  for (int i = mat.size() - 1; i >= 0; --i) {
  