Use DOMCompat::getFirstElementChild() now that it is available from Parsoid

There is only one child, so first/last element child are the same, but
it's a little less confusing to use 'first' here.

Follows-Up: Ifeb3b216e898bec1c3eb5917262820c5809fda45
Depends-On: I753bbbfaf99fb486384b0fa97de71159abb504b3
Change-Id: I65873c117cbd3e1d6efd82797c7b49bf707f5c3a

src/Parsoid/ErrorUtils.php

@@ -54,7 +54,7 @@ class ErrorUtils {
 	): DocumentFragment {
 		$extApi->addTrackingCategory( 'cite-tracking-category-cite-error' );
 		$fragment = $extApi->createInterfaceI18nFragment( 'cite_error', [ $error ] );
-		$fragSpan = DOMCompat::getLastElementChild( $fragment );
+		$fragSpan = DOMCompat::getFirstElementChild( $fragment );
 		DOMUtils::addAttributes( $fragSpan, [ 'class' => 'error mw-ext-cite-error' ] );
 		return $fragment;
 	}