Space between input and button

Add space between input and button in bootstrap

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<div class="container">
    <div class="row">
        <div class="col-md-12">
            <h1>Example</h1>
            
            <div class="input-group">
               <input type="text" class="form-control">
               <span class="input-group-btn">
                    <button class="btn btn-default" type="button">Go!</button>
               </span>
            </div>
        </div>
    </div>
</div>

CSS

.custom-group {
    margin-left: 2px;
}