rust: platform: fix docs related to missing Markdown code spans

Convert `TODO` from documentation to a normal comment, and put code in
block.

This was found using the Clippy `doc_markdown` lint, which we may want
to enable.

Fixes: 683a63b ("rust: platform: add basic platform device / driver abstractions")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250324210359.1199574-9-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/platform.rs

@@ -151,10 +151,11 @@ macro_rules! module_platform_driver {
 ///```
 pub trait Driver: Send {
     /// The type holding driver private data about each device id supported by the driver.
-    ///
-    /// TODO: Use associated_type_defaults once stabilized:
-    ///
-    /// type IdInfo: 'static = ();
+    // TODO: Use associated_type_defaults once stabilized:
+    //
+    // ```
+    // type IdInfo: 'static = ();
+    // ```
     type IdInfo: 'static;
 
     /// The table of OF device ids supported by the driver.