Documentation/admin-guide: use code-block with proper language

Now that we don't have automatic syntax highlighting, use the code-block
directive with the explicitly selected language, where appropriate.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
diff --git a/Documentation/admin-guide/mono.rst b/Documentation/admin-guide/mono.rst
index 9a9744c..cdddc09 100644
--- a/Documentation/admin-guide/mono.rst
+++ b/Documentation/admin-guide/mono.rst
@@ -31,7 +31,9 @@
    more about the configuration process.
 
 3) Add the following entries to ``/etc/rc.local`` or similar script
-   to be run at system startup::
+   to be run at system startup:
+
+   .. code-block:: sh
 
     # Insert BINFMT_MISC module into the kernel
     if [ ! -e /proc/sys/fs/binfmt_misc/register ]; then