doc: fix broken link in vm.md

PR-URL: https://github.com/nodejs/node/pull/7304
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/vm.md

@@ -202,12 +202,12 @@ added: v0.3.1
 * `sandbox` {Object}
 
 If given a `sandbox` object, the `vm.createContext()` method will [prepare
-that sandbox][#vm_what_does_it_mean_to_contextify_an_object] so that it can be
+that sandbox][contextified] so that it can be used in calls to
-used in calls to [`vm.runInContext()`][] or [`script.runInContext()`][]. Inside
+[`vm.runInContext()`][] or [`script.runInContext()`][]. Inside such scripts,
-such scripts, the `sandbox` object will be the global object, retaining all of
+the `sandbox` object will be the global object, retaining all of its existing
-its existing properties but also having the built-in objects and functions any
+properties but also having the built-in objects and functions any standard
-standard [global object][] has. Outside of scripts run by the vm module,
+[global object][] has. Outside of scripts run by the vm module, `sandbox` will
-`sandbox` will remain unchanged.
+remain unchanged.
 
 If `sandbox` is omitted (or passed explicitly as `undefined`), a new, empty
 [contextified][] sandbox object will be returned.