[Merge if needed] Edited ex 11.15

 I've put some spaces between the "< >" because the markdown syntax will
    destroy the formatting when viewed from a browser.
To test this, just try to view ex 11.15 - the readme for ch 11 on web.

ch11/README.md

@@ -39,11 +39,11 @@ I use `set` when i just need to store the `key`, In other hand, I　would like u
 ## [Exercise 11.14](ex11_14.cpp)
 
 ## Exercise 11.15:
->What are the mapped_type, key_type, and value_type of a map from int to vector<int>?
+>What are the mapped_type, key_type, and value_type of a map from int to vector< int >?
 
-- mapped_type : vector<int>
+- mapped_type : vector< int >
 - key_type : int
-- value_type : std::pair<int, vector<int>>
+- value_type : std::pair< int, vector<int> >
 
 ## Exercise 11.16:
 >Using a map iterator write an expression that assigns a value to an element.