editorconfig: fix markdown line length configuration

Fixes markdown line length configuration setting by moving the comment
into its own line. This is required by the editorconfig specification.

.editorconfig

@@ -8,7 +8,8 @@ trim_trailing_whitespace = true
 [*.md]
 indent_size = 2
 indent_style = space
-max_line_length = 100  # Please keep this in sync with bin/lesson_check.py!
+# Please keep max_line_length in sync with bin/lesson_check.py!
+max_line_length = 100
 
 [*.r]
 max_line_length = 80