Documentation: kselftest: Remove duplicate word

Remove duplicate word "for" in kselftest.txt.
Signed-off-by: NZhiyi Sun <zhiyisun@msn.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/kselftest.txt

@@ -73,7 +73,7 @@ To install selftests in an user specified location:
 Contributing new tests
 ======================
 
-In general, the rules for for selftests are
+In general, the rules for selftests are
 
  * Do as much as you can if you're not root;