add hint what the default is

Signed-off-by: Simon L <[email protected]>

php/templates/containers.twig

@@ -629,7 +629,7 @@
                                 <input type="hidden" name="{{csrf.keys.value}}" value="{{csrf.value}}">
                                 <input class="button" type="submit" value="Submit timezone" onclick="return confirm('Are you sure that this is a valid timezone? Please double check by following the wikipedia article and checking the correct column since if not, it will break the startup since the database will not get correctly initialized and you will end in a startup loop.')" />
                             </form>
-                            You need to make sure that the timezone that you enter is valid. An example is <b>Europe/Berlin</b>. You can get valid values by looking at the 'TZ database name' column of this list: <a href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones#List"><b>click here</b></a>.<br><br>
+                            You need to make sure that the timezone that you enter is valid. An example is <b>Europe/Berlin</b>. You can get valid values by looking at the 'TZ database name' column of this list: <a href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones#List"><b>click here</b></a>. The default is <b>Etc/UTC<b> if nothing is entered.<br><br>
                         {% else %}
                             The timezone for Nextcloud is currently set to <b>{{ timezone }}</b>. You can reset the timezone again by clicking on the button below.<br><br/>
                             <form method="POST" action="/api/configuration" class="xhr">