doc/SUMMARY: fix build for updated mdbook by flattening

Author: astro

doc/src/SUMMARY.md

@@ -11,10 +11,9 @@
   - [MicroVM options reference ⚙️](./microvm-options.md)
 - [Running a MicroVM as a package](./packages.md)
 - [Preparing a host for declarative MicroVMs](./host.md)
-  - [Network setup](./simple-network.md)
-    - [A simple network setup](./simple-network.md)
-    - [Advanced network setup](./advanced-network.md)
-    - [Routed network setup](./routed-network.md)
+  - [A simple network setup](./simple-network.md)
+  - [Advanced network setup](./advanced-network.md)
+  - [Routed network setup](./routed-network.md)
   - [Host systemd services](./host-systemd.md)
   - [Host options reference ⚙️](./host-options.md)
 - [Declarative MicroVMs](./declarative.md)