docs: rust: fix improper rendering in Arch Support page

Fix improper rendering of table cell (empty bullet list) by rendering
as a dash using the backslash escaping mechanism [1].

Link: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#escaping-mechanism

 [1]
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1069


Signed-off-by: Bo-Wei Chen <tim.chenbw@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Fixes: 90868ff9 ("LoongArch: Enable initial Rust support")
Link: https://lore.kernel.org/r/20240324010915.3089934-1-tim.chenbw@gmail.com


[ Reworded slightly title and message; use "Link:" tag. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>