Use builtin without wrapping them in an array

assets/themes/natuurpunt/natuurpunt.json

@@ -27,9 +27,7 @@
   "defaultBackgroundId": "CartoDB.Positron",
   "layers": [
     {
-      "builtin": [
-        "nature_reserve"
-      ],
+      "builtin": "nature_reserve",
       "override": {
         "source": {
           "osmTags": {
@@ -48,9 +46,7 @@
       }
     },
     {
-      "builtin": [
-        "visitor_information_centre"
-      ],
+      "builtin": "visitor_information_centre",
       "override": {
         "source": {
           "osmTags": {
@@ -69,9 +65,7 @@
       }
     },
     {
-      "builtin": [
-        "trail"
-      ],
+      "builtin": "trail",
       "override": {
         "source": {
           "osmTags": {
@@ -100,9 +94,7 @@
       }
     },
     {
-      "builtin": [
-        "toilet"
-      ],
+      "builtin": "toilet",
       "override": {
         "minzoom": "15",
         "source": {
@@ -126,9 +118,7 @@
       }
     },
     {
-      "builtin": [
-        "birdhide"
-      ],
+      "builtin": "birdhide",
       "override": {
         "minzoom": "15",
         "source": {
@@ -142,9 +132,7 @@
       }
     },
     {
-      "builtin": [
-        "picnic_table"
-      ],
+      "builtin": "picnic_table",
       "override": {
         "minzoom": "16",
         "source": {
@@ -158,9 +146,7 @@
       }
     },
     {
-      "builtin": [
-        "drinking_water"
-      ],
+      "builtin": "drinking_water",
       "override": {
         "minzoom": "16",
         "source": {
@@ -174,9 +160,7 @@
       }
     },
     {
-      "builtin": [
-        "parking"
-      ],
+      "builtin": "parking",
       "override": {
         "minzoom": "16",
         "icon": {
@@ -203,9 +187,7 @@
       }
     },
     {
-      "builtin": [
-        "information_board"
-      ],
+      "builtin": "information_board",
       "override": {
         "minzoom": "16",
         "source": {
@@ -219,9 +201,7 @@
       }
     },
     {
-      "builtin": [
-        "bench"
-      ],
+      "builtin": "bench",
       "override": {
         "minzoom": "18",
         "source": {
@@ -235,9 +215,7 @@
       }
     },
     {
-      "builtin": [
-        "watermill"
-      ],
+      "builtin": "watermill",
       "override": {
         "minzoom": "18",
         "source": {