fix(docs/mapping-types): fix wrong demo code.

get index before push, so the access of this index won't beyond the boundery.

docs/types/mapping-types.rst

@@ -138,8 +138,8 @@ the ``sum`` function iterates over to sum all the values.
             if (keyIndex > 0)
                 return true;
             else {
-                self.keys.push();
                 keyIndex = self.keys.length;
+                self.keys.push();
                 self.data[key].keyIndex = keyIndex + 1;
                 self.keys[keyIndex].key = key;
                 self.size++;