docs: Set key to default value for peanut

As noted in the document, the default name is `ups` and unlikely to change in the future

docs/widgets/services/peanut.md

@@ -19,5 +19,5 @@ Allowed fields: `["battery_charge", "ups_load", "ups_status"]`.
 widget:
   type: peanut
   url: http://peanut.host.or.ip:port
-  key: nameofyourups
+  key: ups
 ```