src: moving f function call comment

The comment about calling the f function seems to have drifted
a little. Moving it to be closer to the actual call.

PR-URL: https://github.com/nodejs/node/pull/8405


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>