doc: use sentence-case for GOVERNANCE.md headers

Use sentence-case in headers per style guide.

PR-URL: https://github.com/nodejs/node/pull/34503


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>