Fix flag change in Dart 3

Author: Tim Sneath

.github/workflows/dart.yml

@@ -53,7 +53,7 @@ jobs:
         run: dart analyze --fatal-warnings
 
       - name: Check documentation can be generated without errors
-        run: dart doc --dry-run --validate-links
+        run: dart doc --validate-links
 
       - name: Run tests on generation tool
         working-directory: tool/win32gen