docs: Fix markup that led to unreadable example

We used XML to markup when we should have used our own brand of markdown
instead. This fixes the example being unreadable unless we trimmed the
XML away from it.

gio/gioenums.h

@@ -1980,13 +1980,10 @@ typedef enum {
  *
  * Note that because new values might be added, it is recommended that applications check
  * #GMemoryMonitorWarningLevel as ranges, for example:
- * <example>
- *  <title>Comparing memory warning levels</title>
- *  <programlisting>
+ * |[<!-- language="C" -->
  * if (warning_level > G_MEMORY_MONITOR_WARNING_LEVEL_LOW)
  *   drop_caches ();
- *  </programlisting>
- * </example>
+ * ]|
  *
  * Since: 2.64
  */