README: use Markdown for horizontal line.

greenrobot-team · greenrobot-team · commit 34ee175d4f37 · 2022-09-07T10:47:09.000+02:00
diff --git a/README.md b/README.md
@@ -23,7 +23,7 @@ ObjectBox Python API
 - [Feedback](#feedback)
 - [License](#license)
 
-<hr/>
+---
 
 Getting started
 ---------------
