SAMD51 typo fix (#14885)

Marlin/src/HAL/HAL_SAMD51/inc/SanityCheck.h

 #endif
 
 #if ENABLED(EMERGENCY_PARSER)
-  #error "EMERGENCY_PARSER is not yet implemented for STM32F4. Disable EMERGENCY_PARSER to continue."
+  #error "EMERGENCY_PARSER is not yet implemented for SAMD51. Disable EMERGENCY_PARSER to continue."
 #endif
 
 #if ENABLED(SDIO_SUPPORT)