update placeholder.txt contents based on review comment

Author: jepler

supervisor/shared/filesystem.c

@@ -145,7 +145,8 @@ bool filesystem_init(bool create_allowed, bool force_create) {
         res = f_mkdir(&vfs_fat->fatfs, "/sd");
         #if CIRCUITPY_FULL_BUILD
         MAKE_FILE_WITH_OPTIONAL_CONTENTS(&vfs_fat->fatfs, "/sd/placeholder.txt",
-            "This placeholder file allows mounting an SD card at /sd\n");
+            "SD cards are mounted at /sd and will shadow this file from Python."
+            " USB will not see the SD card contents here.\n");
         #endif
         #endif