Documentation: kselftest: Remove duplicate word

Remove duplicate word "for" in kselftest.txt.

Signed-off-by: Zhiyi Sun <zhiyisun@msn.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/kselftest.txt b/Documentation/kselftest.txt
index 9bbbcdc..979eaca 100644
--- a/Documentation/kselftest.txt
+++ b/Documentation/kselftest.txt
@@ -73,7 +73,7 @@
 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;