Fix typo in directive schema introspection description.

Author: jhgg

graphql/core/type/introspection.py

@@ -53,7 +53,7 @@
 
 __Directive = GraphQLObjectType(
     '__Directive',
-    description='A Directives provides a way to describe alternate runtime execution and '
+    description='A Directive provides a way to describe alternate runtime execution and '
                 'type validation behavior in a GraphQL document.'
                 '\n\nIn some cases, you need to provide options to alter GraphQL\'s '
                 'execution behavior in ways field arguments will not suffice, such as '

tests/core_type/test_introspection.py

@@ -454,7 +454,7 @@ def test_executes_an_introspection_query():
                                    'name': u'__EnumValue',
                                    'possibleTypes': None},
                                {
-                                   'description': u"A Directives provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.\n\nIn some cases, you need to provide options to alter GraphQL's execution behavior in ways field arguments will not suffice, such as conditionally including or skipping a field. Directives provide this by describing additional information to the executor.",
+                                   'description': u"A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.\n\nIn some cases, you need to provide options to alter GraphQL's execution behavior in ways field arguments will not suffice, such as conditionally including or skipping a field. Directives provide this by describing additional information to the executor.",
                                    'enumValues': None,
                                    'fields': [{'args': [],
                                                'deprecationReason': None,