fix: specify the slnx file

Co-authored-by: André Silva <2493377+askpt@users.noreply.github.com>
Signed-off-by: Weihan Li <weihanli@outlook.com>

Author: WeihanLi

.github/workflows/dotnet-format.yml

@@ -20,4 +20,4 @@ jobs:
           dotnet-version: 9.0.x
 
       - name: dotnet format
-        run: dotnet format --verify-no-changes
+        run: dotnet format --verify-no-changes OpenFeature.slnx