[dap] Fix link to Dart DAP info

This readme was moved from DDS to third_party/pkg/dap but this link still points to the old location (now a 404).

Change-Id: I737d74085ea726e746a837537121a07bce7e4053
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/378712
Reviewed-by: Helin Shiah <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>

Author: DanTup

third_party/pkg/dap/README.md

@@ -12,4 +12,4 @@ To regenerate the generated code, run the script in "tool/dap/generate_all.dart"
 with no arguments. To download the latest version of the specification before
 regenerating the code, run the same script with the "--download" argument.
 
-More information on Dart support for DAP is available [here](https://github.com/dart-lang/sdk/blob/main/pkg/dds/tool/dap/README.md).
+More information on Dart support for DAP is available [here](https://github.com/dart-lang/sdk/blob/main/third_party/pkg/dap/tool/README.md).