app/views/notification_mailer/updated_event.html.erb
<p>
Hello <%= @user.name %>,
</p>
<p>
The following information was updated in <%= @event.title %>:
</p>
<ul>
<% @updated_data.each do |data| %>
<li><%= data.capitalize %></li>
<% end %>
</ul>
<p>
Go to event: <%= link_to @event.title, group_event_url(@event.group, @event) %>
</p>