docs: Highlight Ctrl-r keybinding on docs page (#3489)

I had to look 5 times and I think highlighting it like this might've
reduced it to 2 :)

<!-- Thank you for making a PR! Bug fixes are always welcome, but if
you're adding a new feature or changing an existing one, we'd really
appreciate if you open an issue, post on the forum, or drop in on
Discord -->

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

docs/docs/guide/basic-usage.md

@@ -15,11 +15,11 @@ While you work, Atuin records:
 
 ## Opening and using the TUI
 
-At any time, you can open the TUI with the default keybindings of the up arrow, or ctrl-r.
+At any time, you can open the TUI with the default keybindings of the up arrow, or `ctrl-r`.
 
 Once in the TUI, press enter to immediately execute a command, or press tab to insert it into your shell for editing.
 
-While searching in the TUI, you can adjust the "filter mode" by repeatedly pressing ctrl-r. Atuin can filter by:
+While searching in the TUI, you can adjust the "filter mode" by repeatedly pressing `ctrl-r`. Atuin can filter by:
 
 1. All hosts
 2. Just your local machine