[css-inline-3] Fix markup

Author: fantasai

css-inline-3/Overview.bs

@@ -515,7 +515,7 @@ Creating Initial Letters: the 'initial-letters' property</h3>
 
 		<dt><dfn><<number>></dfn>
 		<dd>
-			This first argument defines the <dfn dfn lt="initial letter size">size</a>
+			This first argument defines the <dfn dfn lt="initial letter size">size</dfn>
 			of the initial letter
 			in terms of how many lines it occupies.
 			Values less than one are invalid.