Instruction to generate a new theme

Problem/Motivation

In README.md, the Generating a New Theme instruction places the new theme in the wrong directory: themes rather than themes/custom

Steps to reproduce

Navigate to the contrib prototype theme folder and run the suggested
php generator.php -n custom_theme -d 'Custom Theme' -p themes -a short

Proposed resolution

Replace thee suggestion with
php generator.php -n custom_theme -d 'Custom Theme' -p themes/custom -a short