You can't fix the issue by deleting it in your cstrike maps folder and redownloading it, since de_dust2 is a stock map and it's in your
cache, not your cstrike folder.
However, you can still fix it by verifying your game cache:
First, open Steam and go to the Counter-Strike Source entry.
Right click the entry in the list and select "Properties".
Go to the "Local Files" tab and click on "Verify integrity of game cache..."
Your game will validate. Once it is complete, you will get a window telling you if all your files were complete/undamaged. You can just close this window and go back to your main Steam window. If any files failed to validate, do not worry, as Steam will automatically download and repair the files in question.
If you still have issues after completing this, you will likely have to reinstall Counter-Strike: Source.