SUMMARY: move link to v6.8 documentation

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

Author: ojeda

SUMMARY.md

@@ -68,7 +68,7 @@
 
   - [Kernel documentation (mainline)](https://docs.kernel.org/rust/)
   - [Kernel documentation (next)](https://docs.kernel.org/next/rust/)
-  - [`rustdoc` code docs (v6.8-rc3)](https://rust-for-linux.github.io/docs/v6.8-rc3/kernel/)
+  - [`rustdoc` code docs (v6.8)](https://rust-for-linux.github.io/docs/v6.8/kernel/)
   - [`rustdoc` code docs (`rust` 2023-03-13)](https://rust-for-linux.github.io/docs/rust/kernel/)
   - [Out-of-tree module template](https://github.com/Rust-for-Linux/rust-out-of-tree-module)