doc: replace require() with reference links in http2.md
Refs: https://github.com/nodejs/node/pull/34929#discussion_r477101289
Checklist
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documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Refs: https://github.com/nodejs/node/pull/34929#discussion_r477101289