Add missing BEING/END to PL/pgSQL function.

And add a comment about the hypothetical `users` table. Resolves #163.

Author: theory

doc/pgtap.mmd

@@ -305,17 +305,19 @@ What a sweet unit!
 
 If you're used to xUnit testing frameworks, you can collect all of your tests
 into database functions and run them all at once with `runtests()`. This is
-similar to how [PGUnit](http://en.dklab.ru/lib/dklab_pgunit/)s. The
+similar to how [PGUnit](http://en.dklab.ru/lib/dklab_pgunit/) works. The
 `runtests()` function does all the work of finding and running your test
 functions in individual transactions. It even supports setup and teardown
 functions. To use it, write your unit test functions so that they return a set
 of text results, and then use the pgTAP assertion functions to return TAP
-values, like so:
+values. Here's an example, testing a hypothetical `users` table:
 
     CREATE OR REPLACE FUNCTION setup_insert(
     ) RETURNS SETOF TEXT AS $$
+    BEGIN
         RETURN NEXT is( MAX(nick), NULL, 'Should have no users') FROM users;
         INSERT INTO users (nick) VALUES ('theory');
+    END;
     $$ LANGUAGE plpgsql;
 
     CREATE OR REPLACE FUNCTION test_user(