Issue #2839951 by chiranjeeb2410, badjava: API docs...

Issue #2839951 by chiranjeeb2410, badjava: API docs FormattableMarkup::placeholderFormat %variable example contain errors

core/lib/Drupal/Component/Render/FormattableMarkup.php

@@ -147,7 +147,7 @@ public function jsonSerialize() {
    *     A call like:
    *     @code
    *       $string = "%output_text";
-   *       $arguments = ['output_text' => 'text output here.'];
+   *       $arguments = ['%output_text' => 'text output here.'];
    *       $this->placeholderFormat($string, $arguments);
    *     @endcode
    *     makes the following HTML code: