MDL-60025 mod_book: Return correct values for chapters update time

Instead returning the time when the chapter was created or modified we
were returning the time when the book was created or modified.

mod/book/lib.php

@@ -579,8 +579,8 @@ function book_export_contents($cm, $baseurl) {
         $chapterindexfile['filesize']     = 0;
         $chapterindexfile['fileurl']      = moodle_url::make_webservice_pluginfile_url(
                     $context->id, 'mod_book', 'chapter', $chapter->id, '/', 'index.html')->out(false);
-        $chapterindexfile['timecreated']  = $book->timecreated;
-        $chapterindexfile['timemodified'] = $book->timemodified;
+        $chapterindexfile['timecreated']  = $chapter->timecreated;
+        $chapterindexfile['timemodified'] = $chapter->timemodified;
         $chapterindexfile['content']      = format_string($chapter->title, true, array('context' => $context));
         $chapterindexfile['sortorder']    = 0;
         $chapterindexfile['userid']       = null;