fix(cpp). update hash_map (#800)

* fix(cpp). update hash_map

遍历 map 还是 k-v 键值对

* Update hash_map.md

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/cpp/chapter_hashing/hash_map.cpp

@@ -39,13 +39,13 @@ int main() {
     }
 
     cout << "\n单独遍历键 Key" << endl;
-    for (auto key : map) {
+    for (auto kv : map) {
-        cout << key.first << endl;
+        cout << kv.first << endl;
     }
 
     cout << "\n单独遍历值 Value" << endl;
-    for (auto val : map) {
+    for (auto kv : map) {
-        cout << val.second << endl;
+        cout << kv.second << endl;
     }
 
     return 0;

docs/chapter_hashing/hash_map.md

@@ -299,12 +299,12 @@
         cout << kv.first << " -> " << kv.second << endl;
     }
     // 单独遍历键 key
-    for (auto key: map) {
+    for (auto kv: map) {
-        cout << key.first << endl;
+        cout << kv.first << endl;
     }
     // 单独遍历值 value
-    for (auto val: map) {
+    for (auto kv: map) {
-        cout << val.second << endl;
+        cout << kv.second << endl;
     }
     ```