[cssom] Correct typos in serialization step

Author: jugglinmike

cssom-1/Overview.bs

@@ -1561,7 +1561,7 @@ To <dfn export>serialize a CSS rule</dfn>, perform one of the following in accor
    <li>The string "<code>@media</code>", followed by a single SPACE (U+0020).</li>
    <li>The result of performing <a>serialize a media query list</a> on rule's media query list.</li>
    <li>A single SPACE (U+0020), followed by the string "{", i.e., LEFT CURLY BRACKET (U+007B), followed by a newline.</li>
-   <li>The result of performing <a>serialize a CSS rule</a> on the each rule, seperated by a newline and indented by two spaces, in rule's CSSRules list.</li>
+   <li>The result of performing <a>serialize a CSS rule</a> on each rule, separated by a newline and indented by two spaces, in rule's CSSRules list.</li>
    <li>A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)</li>
   </ol>