Fix docs on return type of execute (graphql#439)

API Reference incorrectly states that return type of `execute` is always a `Promise`

Author: brabeji

site/graphql-js/APIReference-Execution.md

@@ -38,7 +38,9 @@ export function execute(
   contextValue?: mixed,
   variableValues?: ?{[key: string]: mixed},
   operationName?: ?string
-): Promise<ExecutionResult>
+): MaybePromise<ExecutionResult>
+
+type MaybePromise<T> = Promise<T> | T;
 
 type ExecutionResult = {
   data: ?Object;