Update README to add php 7.3 switcher

Author: nafiesl

README.md

@@ -57,6 +57,7 @@ Once setup is complete, you can easily switch between different PHP versions usi
 ./switch-to-php-7.0.sh
 ./switch-to-php-7.1.sh
 ./switch-to-php-7.2.sh
+./switch-to-php-7.3.sh
 ```
 
 This will alter the versions of PHP running in Apache and the command line. Your password will be required to enable/disable Apache modules, restart the web server and alter the default PHP CLI version.