Update Clojure example to be more idiomatic

Richard Loveland (rmloveland) commented:

Just ran through the examples as part of reviewing cockroachdb/docs#5028.

We should consider making the instructions a bit more idiomatic for Clojure users by:

- Generating the project with `lein new`, which will use standard Clojure project layouts/filenames (currently we are not, as well as overwriting `test.clj` twice, which is odd)

- The above may necessitate updating the `project.clj` and the code samples just a bit so they build with `lein run` in the new layout

- Updating sample code to follow Lisp style (no dangling parens, etc.)

Jira Issue: DOC-289