[PATCH] Mention service key word more prominently in pg_service.conf docs

Hi hackers,

I just noticed that the docs for pg_service.conf
(https://www.postgresql.org/docs/current/libpq-pgservice.html) don't
mention the actual key word to use in the libpq connection string until
the example in the last sentence, but it mentions the env var in the
first paragraph.  Here's a patch to make the key word equally prominent.

- ilmari