chore: upgrade readme to jakarta mail

Author: Alan5142

appengine-java21/ee8/mail/README.md

@@ -1,13 +1,13 @@
 # JavaMail API Email Sample for Google App Engine Standard Environment
 
-This sample demonstrates how to use [JavaMail][javamail-api] on [Google App Engine
+This sample demonstrates how to use [JakartaMail][jakartamail-api] on [Google App Engine
 standard environment][ae-docs].
 
 See the [sample application documentaion][sample-docs] for more detailed
 instructions.
 
 [ae-docs]: https://cloud.google.com/appengine/docs/java/
-[javamail-api]: https://jakartaee.github.io/mail-api/
+[jakartamail-api]: https://jakartaee.github.io/mail-api/
 [sample-docs]: https://cloud.google.com/appengine/docs/java/mail/
 
 ## Setup