Fix typo (ets-labs#221)

* Fix typo. (ets-labs#214)

* Fix typo

* Update the title tildas

Author: xotonic

docs/introduction/di_in_python.rst

@@ -130,8 +130,8 @@ using :doc:`Dependency Injector <../index>`:
    :language: python
    :linenos:
 
-Userful links
-~~~~~~~~~~~~~
+Useful links
+~~~~~~~~~~~~
 
 There are some useful links related to dependency injection design pattern 
 that could be used for further reading: