[css-writing-modes] Fix bikeshed warning

Author: kojiishi

css-writing-modes/Overview.bs

@@ -28,6 +28,7 @@ Use <i> Autolinks: yes
 <pre class=link-defaults>
 spec:css21; type:property; text:float
 spec:css21; type:property; text:clear
+spec:css-multicol-1; type:property; text:column-gap
 </pre>
 
 <style>