doc: contribute: fix code block for git hook

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 4946132..c7a2d57 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -242,7 +242,7 @@
 style conformity. Checkpatch is available in the scripts directory. To invoke
 it when committing code, edit your *.git/hooks/pre-commit* file to contain:
 
-.. code-block: bash
+.. code-block:: bash
 
     #!/bin/sh
     set -e exec