doc(port_arm): "Traits mode" → "System mode"

This was accidentally changed in commit 7b8979c where the `System`
generic parameter was renamed to `Traits`.

Author: yvt

src/r3_port_arm/src/threading/imp.rs

@@ -84,7 +84,7 @@ impl State {
             asm!("
                 mov r0, sp
 
-                # Switch to Traits mode
+                # Switch to System mode
                 cps #0x1f
 
                 # `dispatch` needs stack