Bump flex preheat temperature

Firmware/variants/1_75mm_MK3S-EINSy10a-E3Dv6full.h

@@ -545,7 +545,7 @@
 #define PET_PREHEAT_HOTEND_TEMP 250
 #define PET_PREHEAT_HPB_TEMP 85
 
-#define FLEX_PREHEAT_HOTEND_TEMP 240
+#define FLEX_PREHEAT_HOTEND_TEMP 250
 #define FLEX_PREHEAT_HPB_TEMP 50
 
 /*------------------------------------