Tweak case light comment

Marlin/Configuration_adv.h

   #define INVERT_CASE_LIGHT false             // Set true if Case Light is ON when pin is LOW
   #define CASE_LIGHT_DEFAULT_ON true          // Set default power-up state on
   #define CASE_LIGHT_DEFAULT_BRIGHTNESS 105   // Set default power-up brightness (0-255, requires PWM pin)
-  //#define CASE_LIGHT_MAX_PWM 128            // Limit pwm
+  //#define CASE_LIGHT_MAX_PWM 128            // Limit PWM duty cycle (0-255)
   //#define CASE_LIGHT_MENU                   // Add Case Light options to the LCD menu
   //#define CASE_LIGHT_NO_BRIGHTNESS          // Disable brightness control. Enable for non-PWM lighting.
   //#define CASE_LIGHT_USE_NEOPIXEL           // Use NeoPixel LED as case light, requires NEOPIXEL_LED.