change for show scroll vertical

Thank you for contributing to vis.js!!

Please make sure to check the following requirements before creating a pull request:

[x] All pull requests must be to the master branch.
[x] Make sure your changes are based on the latest version of the master branch. (Use e.g. git fetch && git rebase origin master to update your feature branch).
[x] Provide an additional or update an example to demonstrate your changes or new features.
[x] Update the documentation if you introduced new behavior or changed existing behavior.
[x] Reference issue numbers of issues that your pull request addresses. (If you write something like fixes #1781 in your git commit message this issue gets closed automatically by merging your pull request).
[x] Expect review comments and change requests by reviewer.

visjs / vis-timeline