docs: update documentation link

use-cases/transactional-templates.md

@@ -1,6 +1,6 @@
 # Transactional Templates
 
-For this example, we assume you have created a [transactional template](https://sendgrid.com/docs/User_Guide/Transactional_Templates/index.html). Following is the template content we used for testing.
+For this example, we assume you have created a [transactional template](https://docs.sendgrid.com/ui/sending-email/create-and-edit-legacy-transactional-templates). Following is the template content we used for testing.
 
 Template ID (replace with your own):