add missing colon of tag key for multiCombo field

this worked in iD as there was a fallback in place to add the colon if missing, but the convention is to always include it in the `key` property for any multiCombo field.

Author: tyrasd

data/fields/plant/output.json

@@ -1,5 +1,5 @@
 {
-    "key": "plant:output",
+    "key": "plant:output:",
     "type": "multiCombo",
     "label": "Form of Power Output",
     "strings": {