[cssom-1] Fix typo

Author: cdoublev

cssom-1/Overview.bs

@@ -1680,7 +1680,7 @@ To <dfn export>parse a CSS rule</dfn> from a string <var>string</var>, run the f
  <li>Let <var>rule</var> be the return value of invoking <a>parse a rule</a> with <var>string</var>.
  <li>If <var>rule</var> is a syntax error, return <var>rule</var>.
  <li>Let <var>parsed rule</var> be the result of parsing <var>rule</var> according to the appropriate CSS specifications, dropping parts that are said to be
- ignored. If the whole style rule is dropped, return a syntax error.
+ ignored. If the whole <var>rule</var> is dropped, return a syntax error.
  <li>Return <var>parsed rule</var>.
 </ol>