Update challenge.php

fix issue #189 part 2

templates/challenge.php

@@ -3,7 +3,7 @@ style('twofactor_totp', 'style');
 ?>
 
 <form method="POST" class="totp-form">
-	<input type="tel" minlength="6" maxlength="6" name="challenge" required="required" autofocus autocomplete="off" autocapitalize="off" placeholder="<?php p($l->t('Authentication code')) ?>">
+	<input type="tel" minlength="6" maxlength="10" name="challenge" required="required" autofocus autocomplete="off" autocapitalize="off" placeholder="<?php p($l->t('Authentication code')) ?>">
 	<button type="submit">
 		<span><?php p($l->t('Submit')); ?></span>
 	</button>