Fix typo in configuration guide: 'AssertsDir' -> 'AssetsDir'.

Author: benjaminbecker

site/guide/9-configuration.md

@@ -185,7 +185,7 @@ assets_dir = "prod_assets/"
 The following code will:
 
   1. Read the configuration parameter in an ad-hoc `attach` fairing.
-  2. Store the parsed parameter in an `AssertsDir` structure in managed state.
+  2. Store the parsed parameter in an `AssetsDir` structure in managed state.
   3. Retrieve the parameter in an `assets` route via the `State` guard.
 
 ```rust