Hint to default data directory

not to before submitted data directory.

Thanks to bartv2 for the heads up.

core/templates/installation.php

@@ -63,7 +63,7 @@
 		<div id="datadirContent">
 			<label for="directory"><?php p($l->t( 'Data folder' )); ?></label>
 			<input type="text" name="directory" id="directory"
-				placeholder="<?php p(OC_Helper::init_var('directory', $_['directory'])); ?>"
+				placeholder="<?php p(OC::$SERVERROOT."/data"); ?>"
 				value="<?php p(OC_Helper::init_var('directory', $_['directory'])); ?>" />
 		</div>
 	</fieldset>