docs: fix extra backtick in Arch Linux install code block

The Arch Linux installation code block in README.md was closed with
four backticks (````) instead of three (```), causing incorrect Markdown
rendering. This removes the extra backtick.

Author: deeferentleeg

README.md

@@ -68,7 +68,7 @@ or Arch Linux
 
 ```
 sudo pacman -S python-lsp-server
-````
+```
 
 Only on Alpine Linux the package is named differently. You can install it there by typing this command in your terminal: