doc: update link for landing PRs

The onboarding doc links to the "Technical How-to" portion of the
Collaborator's Guide for landing PRs. That section is a subsection of
the "Landing Pull Requests" portion of that document. By skipping the
main section header, important information is skipped, such as the part
about not using the merge button and descriptions of the metadata
required. Update the link to target the main section and not the
subsection.

PR-URL: https://github.com/nodejs/node/pull/12415
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

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