Add full stop

Matches the use of description in other files
- lib/graphql/types/relay/node_behaviors.rb
- lib/graphql/types/relay/edge_behaviors.rb
- lib/graphql/types/relay/connection_behaviors.rb

lib/graphql/schema/resolver/has_payload_type.rb

@@ -91,7 +91,7 @@ def generate_payload_type
           resolver_fields = all_field_definitions
           Class.new(object_class) do
             graphql_name("#{resolver_name}Payload")
-            description("Autogenerated return type of #{resolver_name}")
+            description("Autogenerated return type of #{resolver_name}.")
             resolver_fields.each do |f|
               # Reattach the already-defined field here
               # (The field's `.owner` will still point to the mutation, not the object type, I think)