Fix @type in naming-main-types

The type `t` doesn't match the default struct `%__MODULE__{name: nil, params: []}`. This adds `nil` to the possible types. An alternative would be to default `name` to an empty string.

Author: jclem

README.md

@@ -918,7 +918,7 @@ directives (see [Modules](#modules)).
   defstruct name: nil, params: []
 
   @type t :: %__MODULE__{
-    name: String.t,
+    name: String.t | nil,
     params: Keyword.t
   }
   ```