fix config syntax in example

Author: devalog

fern/products/sdks/overview/swift/configuration.mdx

@@ -12,8 +12,9 @@ groups:
     generators:
       - name: fernapi/fern-swift-sdk
         version: <Markdown src="/snippets/version-number-swift.mdx"/>
-        clientClassName: YourClientName
-        environmentEnumName: YourCustomEnvironment
+        config:
+          clientClassName: YourClientName
+          environmentEnumName: YourCustomEnvironment
 ```
 
 ## SDK Configuration Options