[css-rhythm] Fix bikeshed warning for link-defaults

Author: kojiishi

css-rhythm/Overview.bs

@@ -23,6 +23,8 @@ Abstract: This module contains CSS features for aligning content size
 spec:css21; type:property; text:max-height
 spec:css21; type:property; text:max-width
 spec:css21; type:property; text:min-width
+spec:css-writing-modes-4; type:dfn; text:end
+spec:css-writing-modes-4; type:dfn; text:start
 </pre>
 
 Introduction {#intro}