CONTRIBUTING: Fix typo.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 2a3859a..3f5e5aa 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -44,5 +44,5 @@
 
 using your real name (sorry, no pseudonyms or anonymous contributions.)
 
-git commit --signoff will add such a Signed-off-by line by at the end of
+git commit --signoff will add such a Signed-off-by line at the end of
 the commit log message for you.