switch out mountpoint

Signed-off-by: oppiliappan <[e-mail]>

readme.txt

···
 usage
 -----
 
-  cargo run -- <DID/handle>
+  cargo run -- DID|handle

src/main.rs

···
     let mountpoint = matches
         .get_one::<PathBuf>("mountpoint")
         .map(ToOwned::to_owned)
-        .unwrap_or(PathBuf::from(&handle));
+        .unwrap_or(PathBuf::from("mnt"));
     let _ = std::fs::create_dir_all(&mountpoint);
     let resolver = resolver::id_resolver();
     let id = rt.block_on(async {