rust: Kconfig: fix typo on optimization level

Reported-by: bjorn3 <bjorn3_gh@protonmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

lib/Kconfig.debug

@@ -2703,7 +2703,7 @@ config RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C
 
 	      -O2 is currently mapped to -Copt-level=2
 	      -O3 is currently mapped to -Copt-level=3
-	      -Os is currently mapped to -Copt-level=z
+	      -Os is currently mapped to -Copt-level=s
 
 	  The mapping may change over time to follow the intended semantics
 	  of the choice for C as sensibly as possible.