Fix tabbability of 'Storage & database' in install, open on enter stilll missing

Signed-off-by: Jan-Christoph Borchardt <hey@jancborchardt.net>

core/templates/installation.php

@@ -57,7 +57,7 @@ script('core', [
 
 	<?php if(!$_['directoryIsSet'] OR !$_['dbIsSet'] OR count($_['errors']) > 0): ?>
 	<fieldset id="advancedHeader">
-		<legend><a id="showAdvanced"><?php p($l->t( 'Storage & database' )); ?> <img src="<?php print_unescaped(image_path('', 'actions/caret-white.svg')); ?>" /></a></legend>
+		<legend><a id="showAdvanced" tabindex="0"><?php p($l->t( 'Storage & database' )); ?> <img src="<?php print_unescaped(image_path('', 'actions/caret-white.svg')); ?>" /></a></legend>
 	</fieldset>
 	<?php endif; ?>