make clippy happy - implicit_clone

warning: implicitly cloning a `Vec` by calling `to_owned` on its dereferenced type
   --> src/cmd/excel.rs:262:21
    |
262 |     let sheet_vec = sheet_names.to_owned();
    |                     ^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `sheet_names.clone()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implicit_clone
    = note: `-W clippy::implicit-clone` implied by `-W clippy::pedantic`

[skip ci]

src/cmd/excel.rs

@@ -259,7 +259,7 @@ pub fn run(argv: &[&str]) -> CliResult<()> {
     }
     let num_sheets = sheet_names.len();
     #[allow(clippy::redundant_clone)]
-    let sheet_vec = sheet_names.to_owned();
+    let sheet_vec = sheet_names.clone();
 
     let mut wtr = Config::new(&args.flag_output)
         .flexible(args.flag_flexible)