rust: io: fix devres test with new io accessor functions

Fix doctest of `Devres` which still used `writeb` instead of `write8`.

Fixes: 354fd6e ("rust: io: rename `io::Io` accessors")
Signed-off-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20250224-rust-iowrite-read8-fix-v1-1-c6abee346897@kloenk.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: kloenk

rust/kernel/devres.rs

@@ -92,7 +92,7 @@ struct DevresInner<T> {
 /// let devres = Devres::new(&dev, iomem, GFP_KERNEL)?;
 ///
 /// let res = devres.try_access().ok_or(ENXIO)?;
-/// res.writel(0x42, 0x0);
+/// res.write8(0x42, 0x0);
 /// # Ok(())
 /// # }
 /// ```