#(18009) Update syntax mistake in 12.4

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