[first-timers-only] soften 'drive-by' phrasing in CONTRIBUTING.md

[rohitsux]

Looking for a friendly first contribution? CONTRIBUTING.md has the phrase "drive-by PRs" — let's make this more welcoming. Replace "drive-by PRs that change architecture" with "PRs that change architecture without prior discussion".

This is a one-line change. Step-by-step:

1. Click the pencil icon at top-right of CONTRIBUTING.md to edit it on github.com
2. Find the line containing "drive-by PRs"
3. Replace as described above
4. Commit with message: docs(contributing): soften 'drive-by' phrasing
5. Open a PR

Welcome to open source!