minor multi volume config typo (#21880)

Marlin/Configuration_adv.h

   #if ENABLED(MULTI_VOLUME)
     #define VOLUME_SD_ONBOARD
     #define VOLUME_USB_FLASH_DRIVE
-    #define DEFAULT_VOLUME SD_ONBOARD
-    #define DEFAULT_SHARED_VOLUME USB_FLASH_DRIVE
+    #define DEFAULT_VOLUME SV_SD_ONBOARD
+    #define DEFAULT_SHARED_VOLUME SV_USB_FLASH_DRIVE
   #endif
 
 #endif // SDSUPPORT