Fix clippy errors.

src/compare/diff.rs

@@ -2153,7 +2153,7 @@ fn compare_plain_text<'b, 's>(
     let text = emacs.as_text()?;
     let start_ind: usize = text
         .properties
-        .get(0)
+        .first()
         .expect("Should have start index.")
         .as_atom()?
         .parse()?;