[gtk/ebassi/gidocgen] spinbutton: Use markdown for css tree

[Matthias Clasen <matthiasc src gnome org>]

commit fec8a6f6e87f7e410fee97b0bbc75fd35db403e1
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Feb 27 20:06:11 2021 -0500

    spinbutton: Use markdown for css tree

 gtk/gtkspinbutton.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/gtk/gtkspinbutton.c b/gtk/gtkspinbutton.c
index 122bfa65cd..8063981b78 100644
--- a/gtk/gtkspinbutton.c
+++ b/gtk/gtkspinbutton.c
@@ -157,23 +157,23 @@
  *
  * # CSS nodes
  *
- * |[<!-- language="plain" -->
+ * ```
  * spinbutton.horizontal
  * ├── text
  * │    ├── undershoot.left
  * │    ╰── undershoot.right
  * ├── button.down
  * ╰── button.up
- * ]|
+ * ```
  *
- * |[<!-- language="plain" -->
+ * ```
  * spinbutton.vertical
  * ├── button.up
  * ├── text
  * │    ├── undershoot.left
  * │    ╰── undershoot.right
  * ╰── button.down
- * ]|
+ * ```
  *
  * `GtkSpinButton`s main CSS node has the name spinbutton. It creates subnodes
  * for the entry and the two buttons, with these names. The button nodes have