doc: correct default values in http2 docs

The http2 docs include invalid JavaScript values as defaults for three
options. The intention was increased clarity, but we specify valid
values everywhere else in our docs, so let's do that for consistency.

PR-URL: https://github.com/nodejs/node/pull/33997


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>