docs: rust: quick-start: add Debian Testing

Debian Testing is now also providing recent Rust releases (outside of
the freeze period), like Debian Unstable (Sid).

Thus add it to the list.

Cc: Fabian Grünbichler <debian@fabian.gruenbichler.email>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/rust/quick-start.rst

@@ -39,8 +39,8 @@ of the box, e.g.::
 Debian
 ******
 
-Debian Unstable (Sid), outside of the freeze period, provides recent Rust
-releases and thus it should generally work out of the box, e.g.::
+Debian Testing and Debian Unstable (Sid), outside of the freeze period, provide
+recent Rust releases and thus they should generally work out of the box, e.g.::
 
 	apt install rustc rust-src bindgen rustfmt rust-clippy