doc/guide/dts: Clean up DTS template

Add a missing semicolon after /dts-v1/, and a blank line between
/dts-v1/ and the root node.

Use 'code-block:: none' instead of 'code-block:: yaml' as well.
DTS is not YAML.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

doc/guides/dts/index.rst

@@ -195,9 +195,10 @@ compatible that denotes the unique board described by the .dts file.
 Device Tree Source File Template
 ================================
 
-.. code-block:: yaml
+.. code-block:: none
+
+  /dts-v1/;
 
-  /dts-v1/
   / {
           model = "Model name for your board";
           compatible = "compatible for your board";