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: Rust-for-Linux#1069
Signed-off-by: Bo-Wei Chen <tim.chenbw@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Fixes: 90868ff ("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>

Author: BWbwchen

Documentation/rust/arch-support.rst

@@ -16,7 +16,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
 ``arm64``      Maintained        Little Endian only.
-``loongarch``  Maintained        -
+``loongarch``  Maintained        \-
 ``um``         Maintained        ``x86_64`` only.
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================