Update littlefsbuilder.py (#6774)

add a comment to make it clear, since platformIO still creating the `spiffs.bin` for filesystem image. The file name `spiffs.bin` is so much CONFUSING! it makes the user fill the LittleFS creating is fail.

libraries/LittleFS/examples/LITTLEFS_PlatformIO/littlefsbuilder.py

 Import("env")
-env.Replace( MKSPIFFSTOOL=env.get("PROJECT_DIR") + '/mklittlefs' )
\ No newline at end of file
+env.Replace( MKSPIFFSTOOL=env.get("PROJECT_DIR") + '/mklittlefs' )  # PlatformIO now believes it has actually created a SPIFFS