doc: grammar fixes in http2.md

PR-URL: https://github.com/nodejs/node/pull/17972


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>