fix(port_arm_m): fix the error message on invalid exception handler registration

Author: yvt

src/r3_port_arm_m/src/rt/imp.rs

@@ -36,7 +36,7 @@ pub const fn make_interrupt_handler_table<Traits: KernelTraits>() -> InterruptHa
             #[cfg(any())]
             assert!(
                 Traits::INTERRUPT_HANDLERS.get(i).is_none(),
-                "registering a handler for a non-internal exception is \
+                "registering a handler for a non-interrupt exception is \
                 disallowed except for SysTick"
             );
         }