Fix spelling of the word useful in cookbook_guidelines rst

docs/documentation_guidelines/cookbook_guidelines.rst

@@ -126,7 +126,7 @@ The ``doctest`` will pick each group snippets and run them independently.
 
 If you don't declare any group, the code snippet will be added to a group named
 ``default``. If instead, you use ``*`` as a group name, the snippet will be used
-in all testing groups, something normally usefull to use in the test setup::
+in all testing groups, something normally useful to use in the test setup::
 
   .. testsetup:: *