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 <[e-mail]>
Closes: https://github.com/Rust-for-Linux/linux/issues/1069
Signed-off-by: Bo-Wei Chen <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Fixes: 90868ff9cade ("LoongArch: Enable initial Rust support")
Link: https://lore.kernel.org/r/[e-mail]
[ Reworded slightly title and message; use "Link:" tag. ]
Signed-off-by: Miguel Ojeda <[e-mail]>

Documentation/rust/arch-support.rst

···
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
 ``arm64``      Maintained        Little Endian only.
-``loongarch``  Maintained        -
+``loongarch``  Maintained        \-
 ``um``         Maintained        ``x86_64`` only.
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================