Update formatError.d.ts

See e.g. the flow definition in https://github.com/graphql/graphql-js/blob/master/src/error/formatError.js, where they are defined as optional.
I'm no GraphQL expert; caveat emptor.

types/graphql/error/formatError.d.ts

@@ -8,11 +8,11 @@ export function formatError(error: GraphQLError): GraphQLFormattedError;
 
 export type GraphQLFormattedError = {
     message: string,
-    locations: Array<GraphQLErrorLocation>,
-    path: Array<string | number>
+    locations?: Array<GraphQLErrorLocation>,
+    path?: Array<string | number>
 };
 
 export type GraphQLErrorLocation = {
     line: number,
     column: number
-};
+};