samples: rust_misc_device: fix markup in top-level docs

samples: rust_misc_device: fix markup in top-level docs

The meaning of /// is to document the thing that comes after it, so
currently the example is documentation for the `use core::pin::Pin;`
statement. To write top-level docs (and have them rendered as such in
the html by rustdoc), use //! instead.

This does not change the contents of the docs at all. The only change is
changing /// to //!.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 8d9b095b8f89 ("samples: rust_misc_device: Provide an example C program to exercise functionality")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://lore.kernel.org/r/20250313-rust_misc_device_tld-v1-1-a519bced9a6d@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>