Update README.md

Author: darrencarlton

appengine/php72/helloworld/README.md

@@ -1,6 +1,6 @@
-# Hello World on App Engine Standard for PHP 7
+# Hello World on App Engine Standard for PHP 7.2 and 7.3
 
 This sample demonstrates how to deploy a *very* basic application to the
-PHP 7 standard environment in Google App Engine.
+PHP 7.2 and 7.3 standard environment in Google App Engine.
 
 ## View the [full tutorial](https://cloud.google.com/appengine/docs/standard/php7/quickstart)