Fix a couple of simple typos

Marlin/SanityCheck.h

  */
 #if ENABLED(SWITCHING_EXTRUDER)
   #if ENABLED(DUAL_X_CARRIAGE)
-    #error "SINGLENOZZLE and DUAL_X_CARRIAGE are incompatible."
+    #error "SWITCHING_EXTRUDER and DUAL_X_CARRIAGE are incompatible."
   #elif EXTRUDERS != 2
     #error "SWITCHING_EXTRUDER requires exactly 2 EXTRUDERS."
   #elif NUM_SERVOS < 1

Marlin/stepper_indirection.h

   #define X2_ENABLE_INIT SET_OUTPUT(X2_ENABLE_PIN)
   #define X2_ENABLE_WRITE(STATE) WRITE(X2_ENABLE_PIN,STATE)
   #define X2_ENABLE_READ READ(X_ENABLE_PIN)
-#endif // DUAL_X_CARRIAGE
+#endif
 
 // Y motor
 #define Y_STEP_INIT SET_OUTPUT(Y_STEP_PIN)