x11 readme: fix image: into image::

Author: cirosantilli

README.adoc

@@ -4517,7 +4517,7 @@ Outcome: <<image-x11>>
 
 [[image-x11]]
 .X11 Buildroot graphical user interface screenshot
-image:x11.png[image]
+image::x11.png[image]
 
 We don't build X11 by default because it takes a considerable amount of time (about 20%), and is not expected to be used by most users: you need to pass the `-x` flag to enable it.