docs: correct yml extension in README

---
{"auto-commit-msg":{"language":"rust","version":"0.4.0-dev","model":"gemini-2.5-flash-lite","response_time":1.24,"execution_time":1.26}}

Author: sestrella

README.md

@@ -22,14 +22,15 @@ system. Unzip the file and place the binary in a location available on your
 `PATH` environment variable.
 
 <details open>
+
 <summary>Instructions for non-Nix users</summary>
 
 ### pre-commit
 
 Add a `.pre-commit-config.yaml` file to your project's root directory with the
 following content:
 
-```yaml
+```yml
 - repo: local
   hooks:
     - id: auto-commit-msg
@@ -41,7 +42,8 @@ following content:
 
 </details>
 
-<details open>
+<details>
+
 <summary>Instructions for Nix users</summary>
 
 ### devenv