[css-timing-1] Fix markup for 'linear' keywoard

This fixes #2032.

Author: birtles

css-timing-1/Overview.bs

@@ -96,7 +96,7 @@ meaning that its [=output progress value=] is equal to the
 [=input progress value=] for all inputs.
 
 The syntax for the [=linear timing function=] is simply the
-<dfn for=single-timing-function>''linear''</dfn> keyword.
+<dfn dfn-type=value for=single-timing-function>linear</dfn> keyword.
 
 
 Cubic B&eacute;zier timing functions {#cubic-bezier-timing-functions}