rust: io: fix broken intra-doc link to missing flags module

There is no `mod flags` in this case, unlike others. Instead, they are
associated constants for the `Flags` type.

Thus reword the sentence to fix the broken intra-doc link, providing
an example of constant and linking to it to clarify which ones we are
referring to.

Fixes: 493fc33 ("rust: io: add resource abstraction")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250722085500.1360401-1-ojeda@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: ojeda

rust/kernel/io/resource.rs

@@ -177,7 +177,8 @@ unsafe impl Sync for Resource {}
 ///
 /// They can be combined with the operators `|`, `&`, and `!`.
 ///
-/// Values can be used from the [`flags`] module.
+/// Values can be used from the associated constants such as
+/// [`Flags::IORESOURCE_IO`].
 #[derive(Clone, Copy, PartialEq)]
 pub struct Flags(c_ulong);