Update syntax mistake

Prior to this commit there was a missing quote mark. This caused the
`puppet parser validate` tool to fail with the error
> puppet parser
validate manifests/init.pp
err: Could not parse for environment production: Could not match
${httpd_pkg}": at /root/master_home/modules/apache/manifests/init.pp:38
err: Try 'puppet help parser validate' for usage
Add default to case statment

Prior to this commit the case statment did not have a default. This
commit adds a default that uses the fail function per the style guide.
This was updated in the exercise and example files guide in pull
request:

https://github.com/puppetlabs/courseware-fundamentals/pull/23
Merge pull request #53 from acidprime/fundamentals/tickets/18009

#(18009) Update syntax mistake in 12.4