updating comments for the sandbox_custom var file

playbooks/secure_example/vars/sandbox_custom.yml

 ---
 # Set a custom LMS_BASE and PREVIEW_LMS_BASE
-# for single-instance setups
-# cms_nginx_port: 80
-# lms_preview_nginx_port: 80
+# for single-instance setups, you may also want to pass overrides for the following:
+# cms_nginx_port: 80 (defaults to 18010)
+# lms_preview_nginx_port: 80 (defaults to 18020)
 # Override LMS_BASE and PREVIEW_LMS_BASE in the cms env config
 
 # These should be overridden with the -e option on the command line