Update requirement for Query type in mutation docs (#1213)

Author: jkimbo

docs/types/mutations.rst

@@ -48,11 +48,7 @@ So, we can finish our schema like this:
     class MyMutations(graphene.ObjectType):
         create_person = CreatePerson.Field()
 
-    # We must define a query for our schema
-    class Query(graphene.ObjectType):
-        person = graphene.Field(Person)
-
-    schema = graphene.Schema(query=Query, mutation=MyMutations)
+    schema = graphene.Schema(mutation=MyMutations)
 
 Executing the Mutation
 ----------------------

graphene/types/schema.py

@@ -381,7 +381,7 @@ class Schema:
     questions about the types through introspection.
 
     Args:
-        query (Type[ObjectType]): Root query *ObjectType*. Describes entry point for fields to *read*
+        query (Optional[Type[ObjectType]]): Root query *ObjectType*. Describes entry point for fields to *read*
             data in your Schema.
         mutation (Optional[Type[ObjectType]]): Root mutation *ObjectType*. Describes entry point for
             fields to *create, update or delete* data in your API.