fix ex11.34

ch11/README.md

@@ -104,8 +104,13 @@ std::pair<std::map<std::string, std::vector<int>>::iterator, bool> // return
 ## Exercise 11.34:
 >What would happen if we used the subscript operator instead of find in the transform function?
 
-If the subscript operator was used instead, it would add the given key into the map when no element with matching key was found.
-
+In gcc 4.8.3, will report error:
+```cpp
+error: passing ‘const std::map<std::basic_string<char>, std::basic_string<char> >’ as ‘this’ argument of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string<char>; _Tp = std::basic_string<char>; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, std::basic_string<char> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::basic_string<char>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]’ discards qualifiers [-fpermissive]
+     auto key = m[s];
+                   ^
+```
+Because std::map's operator is not declared as **const**,but m is declared as a  reference to  std::map with **const**.If insert new pair,it will cause error.
 ## Exercise 11.35:
 >In buildMap, what effect, if any, would there be from rewriting `trans_map[key] = value.substr(1);` as `trans_map.insert({key, value.substr(1)})`?