[dap] Update the repo URL in pkg:dap pubspec

This package was moved to third_party but the URL here was not updated so the link on Pub is a 404.

This will need a new Pub release before it updates, but I'm not sure it's worth doing until there are some changes.

Change-Id: Ibd1e4ba0208f13f27e1e04ba141e8a78155bd2bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/369700
Reviewed-by: Ben Konyi <[email protected]>
Reviewed-by: Helin Shiah <[email protected]>
Commit-Queue: Elliott Brooks <[email protected]>

Author: DanTup

third_party/pkg/dap/pubspec.yaml

@@ -3,7 +3,7 @@ version: 1.3.0
 description: >-
   A package of classes that are generated from the DAP specifications along with
   their generating code.
-repository: https://github.com/dart-lang/sdk/tree/main/pkg/dap
+repository: https://github.com/dart-lang/sdk/tree/main/third_party/pkg/dap
 
 environment:
   sdk: '>=2.19.0 <4.0.0'