minor: Simplify another document_mut statement

helix-term/src/application.rs

@@ -195,7 +195,7 @@ impl Application {
                         // `--vsplit` or `--hsplit` are used, the file which is
                         // opened last is focused on.
                         let view_id = editor.tree.focus;
-                        let doc = editor.document_mut(doc_id).unwrap();
+                        let doc = doc_mut!(editor, &doc_id);
                         let pos = Selection::point(pos_at_coords(doc.text().slice(..), pos, true));
                         doc.set_selection(view_id, pos);
                     }