fixed outdated code

src/main.rs

@@ -198,8 +198,7 @@ impl State {
                     let state = InsertState { append: false };
                     self.cursor.line += 1;
                     self.cursor.column = 0;
-                    let index = self.buffer.cursor_to_char(self.cursor);
-                    self.buffer.text.insert(index, "\n");
+                    self.buffer.insert_char(self.cursor, '\n');
                     self.mode = Mode::Insert(state);
                 }
                 KeyCode::Char(':') => {