Improve reference for Amphitheater preset (#1647)

Author: arch0345

data/presets/amenity/theatre/type/amphi.json

@@ -20,5 +20,9 @@
         "amenity": "theatre",
         "theatre:type": "amphi"
     },
+    "reference": {
+        "key": "theatre:type",
+        "value": "amphi"
+    },
     "name": "Amphitheater"
 }