doc: use sentence case for headers in BUILDING.md

Be consistent with the rest of the file and our doc style guide.

PR-URL: https://github.com/nodejs/node/pull/37250


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>