change readme and make dest argument optional

Author: m0veax

README.md

@@ -42,5 +42,5 @@ fn main() {
 extract .dtb files from images
 
 ```
-$> cargo run extract-dtb
+cargo run extract-dtb
 ```

extract-dtb/main.rs

@@ -13,6 +13,7 @@ struct Args {
     /// Filename
     #[arg(required=true)]
     filename: String,
+    #[arg(required=false)]
     dest: String,
 }