Fix conditions for display strings

Marlin/language_en.h

 #ifndef MSG_HEATING
 #define MSG_HEATING                         "Heating..."
 #endif
-#ifndef MSG_HEATING
+#ifndef MSG_HEATING_COMPLETE
 #define MSG_HEATING_COMPLETE                "Heating done."
 #endif
-#ifndef MSG_HEATING
+#ifndef MSG_BED_HEATING
 #define MSG_BED_HEATING                     "Bed Heating."
 #endif
-#ifndef MSG_HEATING
+#ifndef MSG_BED_DONE
 #define MSG_BED_DONE                        "Bed done."
 #endif