Remove incorrect mutability

src/main.rs

@@ -224,7 +224,7 @@ fn main() {
                 let ft = metadata.file_type();
 
                 if ft.is_file() {
-                        let mut a = FileToProcess {
+                        let a = FileToProcess {
                             name: format!("{}",
                                           entry.path().display()),
                             hash: vec![],