corrects schema version property name in RenderIndex.spec.json (#1224)

Author: heckj

Sources/SwiftDocC/SwiftDocC.docc/Resources/RenderIndex.spec.json

@@ -15,7 +15,7 @@
                     "interfaceLanguages"
                 ],
                 "properties": {
-                    "identifier": {
+                    "schemaVersion": {
                         "$ref": "#/components/schemas/SchemaVersion"
                     },
                     "interfaceLanguages": {