docs: Update transactions how to example

[jasonblanchard]

The example for transactions is a little confusing for a few reasons:

1. It mentions WithTx, but then it never actually calls it
2. Confusing use of the db package - I believe db.Begin(ctx) should be p.Begin(ctx)

My suggestion here does a few things:


1. Fixes the above issues.
2. Uses naming conventions that match the “Getting started with PostgreSQL” tutorial (i.e package tutorial, queries, schema.sql, query.sql, db, etc).
3. Use database/sql and lib/pq to match the “Getting started with PostgreSQL” tutorial.

[kyleconroy]

Thanks for the improvements!