fix header for myst

Author: mmcky

lectures/arellano.md

@@ -1,15 +1,12 @@
 ---
-jupyter:
-  jupytext:
-    text_representation:
-      extension: .md
-      format_name: markdown
-      format_version: '1.3'
-      jupytext_version: 1.13.7
-  kernelspec:
-    display_name: Python 3
-    language: python
-    name: python3
+jupytext:
+  text_representation:
+    extension: .md
+    format_name: myst
+kernelspec:
+  display_name: Python 3
+  language: python
+  name: python3
 ---
 
 (arellano)=