Fix TERN typo (#21229)

Marlin/src/feature/leds/neopixel.cpp

   else {
     for (uint16_t i = 0; i < pixels(); ++i) {
       #ifdef NEOPIXEL_BKGD_LED_INDEX
-        if (i == NEOPIXEL_BKGD_LED_INDEX && TERN(ENABLED(NEOPIXEL_BKGD_ALWAYS_ON), true, color != 0x000000)) {
+        if (i == NEOPIXEL_BKGD_LED_INDEX && TERN(NEOPIXEL_BKGD_ALWAYS_ON, true, color != 0x000000)) {
           set_color_background();
           continue;
         }