docs: fix "a HTTP"/"a SSR" grammar and Node.js casing (#99542)
Small docs fixes in the Pages Router docs:
- `use-router.mdx`: `Node.JS` -> `Node.js`, and `a SSR` -> `an SSR`
- `07-api-routes.mdx`: `a HTTP` -> `an HTTP` (heading and body)
The heading anchor changes from `#sending-a-http-response` to
`#sending-an-http-response`. I checked that nothing in `docs/` links to
the old anchor.
