doc: clarify http.get data consumption requirement

With the previous wording, I read this sentence as meaning, "you
_must_ use the http.get callback mechanism and cannot register a
listener on the returned http.ClientRequest object." This is obviously
not the intention, so adjust the sentence to make this clearer.

PR-URL: https://github.com/nodejs/node/pull/15049


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>