SUMMARY: update topic branches URLs

These worked since GitHub redirected and showed a banner explaining that
they were renamed. While that is very useful (so that people realize the
change happened), it is time to update the links to avoid relying on that.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

SUMMARY.md

@@ -63,8 +63,8 @@
 
 ## Topic branches
 
-  - [`rust-pci`](https://github.com/Rust-for-Linux/linux/tree/rust-pci)
-  - [`rust-net`](https://github.com/Rust-for-Linux/linux/tree/rust-net)
+  - [`staging/rust-pci`](https://github.com/Rust-for-Linux/linux/tree/staging/rust-pci)
+  - [`staging/rust-net`](https://github.com/Rust-for-Linux/linux/tree/staging/rust-net)
 
 ## Documentation