app/views/devise/mailer/unlock_instructions.html.erb
<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html charset=UTF-8">
</head>
<body>
<% if @resource.name.present? %>
<p>Hello <%= @resource.name %>!</p>
<% else %>
<p>Hello!</p>
<% end %>
<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>
</body>
</html>