doc: update vm.runInDebugContext() example

The debugger needs to be active now before one is allowed to query the
list of scripts.  Replace the example with one that works without
installing a debug event listener first.

Fixes: https://github.com/nodejs/node/issues/4862
PR-URL: https://github.com/nodejs/node/pull/6757


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>