Move GenericArgs chapter under ty (#624)

Author: mark-i-m

src/SUMMARY.md

@@ -56,7 +56,7 @@
     - [The `ty` module: representing types](./ty.md)
         - [Generics and substitutions](./generics.md)
         - [`TypeFolder` and `TypeFoldable`](./ty-fold.md)
-    - [Generic arguments](./generic_arguments.md)
+        - [Generic arguments](./generic_arguments.md)
     - [Type inference](./type-inference.md)
     - [Trait solving (old-style)](./traits/resolution.md)
         - [Higher-ranked trait bounds](./traits/hrtb.md)