app/views/devise/mailer/unlock_instructions.html.erb
<p>Hello <%= @resource.email %>!</p>
<p>Your account has been locked due to an excessive number of unsuccessful sign in attempts.</p>
<p>Click the link below to unlock your account:</p>
<p><%= link_to 'Unlock my account', unlock_url(@resource, :unlock_token => @token) %></p>