Fix bad-return-value-key lint in netbox_virtual_machine.py

Author: dchiquito

plugins/modules/netbox_virtual_machine.py

@@ -196,7 +196,7 @@
 """
 
 RETURN = r"""
-virtual machine:
+virtual_machine:
   description: Serialized object as created or already existent within NetBox
   returned: success (when I(state=present))
   type: dict