CONTRIBUTING.md: remove the sourceforge mention

Reported-By: Michael Osipov

CONTRIBUTING.md

@@ -16,11 +16,9 @@ Send your suggestions using one of these methods:
 
  1. in a mail to the mailing list
 
- 2. in the [bug tracker](https://sourceforge.net/p/curl/bugs/)
+ 2. as a pull request on github
 
- 3. as a pull request on github
-
- 4. as an issue on github
+ 3. as an issue on github
    
 
 / The cURL team!