clean index each time

slides/gen_index.sh

@@ -44,7 +44,7 @@ done
 IFS=$'\n'
 classes=($(sort <<<"${classes[*]}"))
 date=$(date '+%Y-%m-%d')
-echo "---" >> index.md
+echo "---" > index.md
 echo "title: Cours de programmation séquentielle" >> index.md
 echo "date: ${date}" >> index.md
 echo "---" >> index.md