display error if directory not specified

src/main.rs

@@ -6,7 +6,13 @@ use std::{
 };
 
 fn main() {
-    let directory = args().nth(1).unwrap();
+    match args().nth(1) {
+        Some(dir) => rename_files_in_directory(dir),
+        None => println!("Please provide a directory"),
+    }
+}
+
+fn rename_files_in_directory(directory: String) {
     println!("Renaming files in {}", directory);
     match rename_files(&directory, &directory) {
         Ok(count) => println!("Renamed {} files", count),