Use exec to replace bash with apache2 after environment is set up

In the example, bash is used to run a script that populates the environment and then to spawn apache2.  The result of the original command is that bash stays running as a child of supervisord, and apache2 is a child of bash.  

This change adds ``exec`` so that after bash does its work of setting up the environment, the bash process is replaced with apache2.  This allows supervisord to manage apache2 directly.  This page isn't intended as a comprehensive guide to configuring Supervisor but making this change wouldn't hurt either.