[css-writing-modes] Fix diagram.

--HG--
extra : rebase_source : 0ef781506086e962728869f42b95bbbf0df17b73

Author: fantasai

css-writing-modes/Overview.bs

@@ -1055,7 +1055,7 @@ Introduction to Vertical Text Layout</h2>
                      region would represent the bi-orientational scripts, while
                      horizontal-only and vertical-only scripts would occupy their
                      respective circles' exclusive regions."
-                src="baseline-align-super.gif">
+                src="diagrams/script-orientations.png">
   </div>
 
   <p>In modern typographic systems, all glyphs are assigned a horizontal