Fix a comment

Marlin/src/lcd/dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp

   #define CPU_ST7920_DELAY_2 DELAY_NS(0)
   #define CPU_ST7920_DELAY_3 DELAY_NS(63)
 #else
-  #error "No valid condition for delays in 'ultralcd_st7920_u8glib_rrd.h'"
+  #error "No valid condition for delays in 'ultralcd_st7920_u8glib_rrd_AVR.h'"
 #endif
 
 #ifndef ST7920_DELAY_1