[css-writing-modes] Fix broken biblio ref.

Author: tabatkins

css-writing-modes/Overview.bs

@@ -2509,7 +2509,7 @@ Full-width Characters</h5>
 
   <p class="note">
     Properties that affect glyph selection,
-    such as the <a property>font-variant</a> and <a property>font-feature-settings</a> properties defined in [CSS3-FONTS],
+    such as the <a property>font-variant</a> and <a property>font-feature-settings</a> properties defined in [[CSS3-FONTS]],
     can potentially affect the selection of variants for characters included in combined text runs.
     Authors are advised to use these properties with care when 'text-combine-upright' is also used.