Fix error in CityJSONFeature description, all the children must be in…

…cluded

changed the sentence + made the snippet with 3 levels

fixes #136

specs/specs.bs

@@ -1424,7 +1424,7 @@ A CityJSON Feature Object:
   - is a JSON object.
   - **must** have one member with the name `"type"`, whose value must be `"CityJSONFeature"`.
   - **must** have one member with the name `"id"`, whose value must be a string representing the identifier of the City Object Feature, this is used to clearly identify which of the CityObjects is the parent.
-  - **must** have one member with the name `"CityObjects"`. The value of this member is a collection of key-value pairs, where the key is the ID of the object, and the value is one City Object. The ID of a City Object should be unique (within one CityJSONFeature Object), and all the children of the `"CityJSONFeature"` (only those one level below the current one) must be listed.
+  - **must** have one member with the name `"CityObjects"`. The value of this member is a collection of key-value pairs, where the key is the ID of the object, and the value is one City Object. The ID of a City Object should be unique (within one `"CityJSONFeature"`), and all the children of the `"CityJSONFeature"` must be included (and the children of the children (recursively), if there are any).
   - **must** have one member with the name `"vertices"`, whose value is an array of coordinates of each vertex of the current City Object Feature (stored with integers). Their position in this array (0-based) is used as an index to be referenced by the Geometry Objects for the JSON object (warning: the vertices are local to the JSON object).
   - **may** have one member with the name `"appearance"`, the value may contain JSON objects representing the textures and/or materials of surfaces. See [[#appearance-object]] for details.
   - **must not** have other members.
@@ -1449,12 +1449,18 @@ A CityJSON Feature Object:
       "attributes": { 
         "roofType": "gabled roof"
       },
+      "children": ["mypart"],
+      "geometry": [...]
+    },
+    "mypart": {
+      "type": "BuildingPart", 
+      "parents": ["id-1"],
       "children": ["mybalcony"],
       "geometry": [...]
     },
     "mybalcony": {
       "type": "BuildingInstallation", 
-      "parents": ["id-1"],
+      "parents": ["mypart"],
       "geometry": [...]
     }
   },