fix progress bar not to break apart

[enklht]

I found an issue with the rendering of progress bars in the section slides. I have added a single line of code to prevent this problem.

Thank you for considering my contribution. Feel free to use or modify it as needed!

minimal example

#import "@preview/touying:0.5.3": *
#import themes.metropolis: *
#show: metropolis-theme
#set grid(gutter: 1em)

= First chapter
= Second chapter

This is how the example above renders in typst.app.
You may see the progress bar is broken in the first chapter title slide.

[OrangeX4]

Thanks!