docs: clarify PR template instructions (#41957)

Running "pnpm lint" on a documentation change won't work unless "pnpm
build" has previously been run, e.g. on a fresh checkout of a fork after
clicking the published docs "edit this on GitHub" button.

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Steven <steven@ceriously.com>

.github/pull_request_template.md

@@ -21,5 +21,5 @@ Choose the right checklist for the change that you're making:
 
 ## Documentation / Examples
 
-- [ ] Make sure the linting passes by running `pnpm lint`
+- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
 - [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)