🩹 Fix STM32 HW Serial 6 (MarlinFirmware#26763)

Followup to MarlinFirmware#26328

Author: thisiskeithb

Marlin/src/HAL/STM32/HardwareSerial.cpp

@@ -151,7 +151,7 @@
   #else
     HAL_HardwareSerial HSerial6(UART6);
   #endif
-  void serialEvent5() __attribute__((weak));
+  void serialEvent6() __attribute__((weak));
 #endif
 
 // Constructors ////////////////////////////////////////////////////////////////