app/views/devise/mailer/confirmation_instructions.html.erb
<% require 'devise/version' %>
<p><%= t('.greeting', :recipient => @resource.email, :default => "Welcome #{@resource.email}!") %></p>
<p><%= t('.instruction', :default => "You can confirm your account email through the link below:") %></p>
<p><%= link_to t('.action', :default => "Confirm my account"),
confirmation_url(@resource, :confirmation_token => (Devise::VERSION.start_with?('3.') ? @token : @resource.confirmation_token)) %></p>