fix: update get element tag name to return lowercase tag name (#1962)

Author: juliandescottes

index.html

@@ -5823,7 +5823,7 @@ <h4><dfn>Get Element Tag Name</dfn></h4>
   with <var>URL variables</var>["<code>element id</code>"].
 
  <li><p>Let <var>qualified name</var> be the result of getting
-  <var>element</var>&apos;s {{Element/tagName}} IDL attribute.
+  <var>element</var>&apos;s {{Element/tagName}} IDL attribute in [=ASCII lowercase=].
 
  <li><p>Return <a>success</a> with data <var>qualified name</var>.
 </ol>