don't use strings for paths

src/main.rs

@@ -1,5 +1,6 @@
 // #![allow(unused_imports)]
 
+use std::path::PathBuf;
 use tokio::net::TcpListener;
 
 use herodb::server;
@@ -13,7 +14,7 @@ use clap::Parser;
 struct Args {
     /// The directory of Redis DB file
     #[arg(long)]
-    dir: String,
+    dir: PathBuf,
 
     /// The port of the Redis server, default is 6379 if not specified
     #[arg(long)]