Fix MKS UI SPI flash typo (#19410)

Marlin/src/lcd/extui/lib/mks_ui/SPIFlashStorage.cpp

   #if HAS_SPI_FLASH_COMPRESSION
     // Restart the compressed buffer, keep the pointers of the uncompressed buffer
     m_compressedDataUsed = 0;
-  #elif
+  #else
     m_pageDataUsed = 0;
   #endif
   m_currentPage++;