fixup: doc add link to ecma262

Author: MylesBorins

doc/api/esm.md

@@ -7,9 +7,9 @@
 
 ## Introduction
 
-ECMAScript modules are the official standard format to package JavaScript code
-for reuse. Modules are defined using a variety of [`import`][] and [`export`][]
-statements.
+ECMAScript modules are [the official standard format][] to package JavaScript
+code for reuse. Modules are defined using a variety of [`import`][] and
+[`export`][] statements.
 
 Node.js fully supports ECMAScript modules as they are currently specified and
 provides limited interoperability between them and the existing module format,
@@ -726,3 +726,4 @@ success!
 [`import()`]: #esm_import-expressions
 [ecmascript-modules implementation]: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md
 [WHATWG JSON modules]: https://github.com/whatwg/html/issues/4315
+[the official standard format]: https://tc39.github.io/ecma262/#sec-modules