Fix PRUSA commands

Firmware/Marlin_main.cpp

@@ -4550,7 +4550,7 @@ void process_commands()
 #elif defined(BOOTAPP) //this is a safety precaution. This is because the new bootloader turns off the heaters, but the old one doesn't. The watchdog should be used most of the time.
             asm volatile("jmp 0x3E000");
 #endif
-		}else if (code_seen_P("fv")) { // PRUSA fv
+        } else if (code_seen_P(PSTR("fv"))) { // PRUSA fv
         // get file version
         #ifdef SDSUPPORT
         card.openFileReadFilteredGcode(strchr_pointer + 3,true);