Fix typo in style.rs (#5845)

lien -> line

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

* Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>
* [ x ] I have followed the instructions in the PR template

Author: TPhoenixyz

crates/egui/src/style.rs

@@ -989,7 +989,7 @@ pub struct Visuals {
     /// Show a background behind collapsing headers.
     pub collapsing_header_frame: bool,
 
-    /// Draw a vertical lien left of indented region, in e.g. [`crate::CollapsingHeader`].
+    /// Draw a vertical line left of indented region, in e.g. [`crate::CollapsingHeader`].
     pub indent_has_left_vline: bool,
 
     /// Whether or not Grids and Tables should be striped by default