chore(docs): update GAE README for PHP 7.4 (GoogleCloudPlatform#1611)

Author: bshaffer

appengine/standard/getting-started/README.md

@@ -1,7 +1,7 @@
-# Getting Started on App Engine for PHP 7.2
+# Getting Started on App Engine for PHP 7.4
 
 This sample demonstrates how to deploy a PHP application which integrates with
-Cloud SQL and Cloud Storage on App Engine Standard for PHP 7.2. The tutorial
+Cloud SQL and Cloud Storage on App Engine Standard for PHP 7.4. The tutorial
 uses the Slim framework.
 
 ## View the [full tutorial](https://cloud.google.com/appengine/docs/standard/php7/building-app/)