Fix clippy lint for unwrap_or_default

Latest nightly clippy warns:

    warning: use of `unwrap_or_else` to construct default value
      --> inquire/src/prompts/multiselect/prompt.rs:67:14
       |
    67 |             .unwrap_or_else(BTreeSet::new);
       |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_default()`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default
       = note: `#[warn(clippy::unwrap_or_default)]` on by default

inquire/src/prompts/multiselect/prompt.rs

@@ -64,7 +64,7 @@ where
                     .filter(|i| *i < mso.options.len())
                     .collect()
             })
-            .unwrap_or_else(BTreeSet::new);
+            .unwrap_or_default();
 
         Ok(Self {
             message: mso.message,