rust: list: Fix typo `much` in arc.rs

rust: list: Fix typo `much` in arc.rs

Correct the typo (s/much/must) in the ListArc documentation.

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1166
Fixes: a48026315cd7 ("rust: list: add tracking for ListArc")
Signed-off-by: Sylvan Smit <sylvan@sylvansmit.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250529162923.434978-1-sylvan@sylvansmit.com
[ Changed tag to "Reported-by" and sorted. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>