[CSSOM] Fix typo in "insert a CSS rule" (#12375)

Author: Calme1709

cssom-1/Overview.bs

@@ -1904,7 +1904,7 @@ follow these steps:
   </ul>
  <li>If <var>new rule</var> is a syntax error, <a>throw</a>
  a {{SyntaxError}} exception.
- <li>If <var>new rule</var> cannot be inserted into <var>list</var> at the zero-index position <var>index</var> due to constraints
+ <li>If <var>new rule</var> cannot be inserted into <var>list</var> at the zero-indexed position <var>index</var> due to constraints
  specified by CSS, then <a>throw</a>
  a {{HierarchyRequestError}} exception. [[!CSS21]]