Add missing yaml to code snipped in yaml_api.md

Signed-off-by: Leonardo Ochoa <lochoa@redhat.com>

Author: leo8a

devel/yaml_api.md

@@ -492,7 +492,7 @@ interfaces:
 
 This is the example of interface with dynamic IP addresses:
 
-```
+```yml
 ---
 interfaces:
   - name: eth1