doc: link and highlight Object.assign

PR-URL: https://github.com/nodejs/node/pull/7670
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/util.md

@@ -651,13 +651,13 @@ Deprecated predecessor of `console.log`.
 
 ### util._extend(obj)
 
-    Stability: 0 - Deprecated: Use Object.assign() instead.
+    Stability: 0 - Deprecated: Use [`Object.assign()`] instead.
 
 The `util._extend()` method was never intended to be used outside of internal
 Node.js modules. The community found and used it anyway.
 
 It is deprecated and should not be used in new code. JavaScript comes with very
-similar built-in functionality through `Object.assign()`.
+similar built-in functionality through [`Object.assign()`].
 
 [`Array.isArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
 [constructor]: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/constructor
@@ -668,3 +668,4 @@ similar built-in functionality through `Object.assign()`.
 [`console.log()`]: console.html#console_console_log_data
 [`console.error()`]: console.html#console_console_error_data
 [`Buffer.isBuffer()`]: buffer.html#buffer_class_method_buffer_isbuffer_obj
+[`Object.assign()`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign