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

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: 493fc33ec252 ("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>