fix: fix maven plugin URL

Author: Alan5142

appengine-java21/ee8/datastore/README.md

@@ -13,7 +13,7 @@ from [Google App Engine standard environment][ae-docs].
 ## Running locally
 
 This example uses the
-[Cloud SDK Maven plugin](https://cloud.google.com/appengine/docs/java/tools/using-maven).
+[Cloud SDK Maven plugin](https://cloud.google.com/appengine/docs/legacy/standard/java/using-maven).
 To run this sample locally:
 
     $ mvn appengine:run