(Documentation) sqlx.In should be used with (?) instead of ($1) for postgres

Hello,

This is a note for people with similar problems in future. I used the sqlx.In helper with a postgres driver,

rows, err := db.Query("SELECT * FROM users WHERE level IN ($);", levels)

and kept getting an error

number of bindVars less than number arguments

I only had 1 bindVar so it was puzzling and it seemed correct because I had to use ($1) ($2) etc syntax for postgres.

But it turns out I had to use (?) instead of postgres syntax for bindVar.

So for anyone having this same error and using postgres (or think that doing it is correct), switch to (?) instead for the bindVars.

If it helps I'd be happy to do a PR to add this as a footnote to the user documentation.