[cssom] Fix typo 'dash first' -> 'dash prefix'

Author: zcorpan

cssom/Overview.bs

@@ -2241,7 +2241,7 @@ The <dfn>IDL attribute to CSS property</dfn> algorithm for <var>attribute</var>,
 <ol>
  <li>Let <var>output</var> be the empty string.
 
- <li>If the <i>dash first</i> flag is set, append "<code>-</code>" (U+002D) to <var>output</var>.
+ <li>If the <i>dash prefix</i> flag is set, append "<code>-</code>" (U+002D) to <var>output</var>.
 
  <li>For each character <var>c</var> in <var>attribute</var>: