Improve placeholder text

generator/generate.html

                     <div class="form-group">
                         <label for="card-background" class="col-sm-2 control-label">Background</label>
                         <div class="col-sm-10">
-                            <input type="text" id="card-background" class="form-control" placeholder="Background image" data-property="background_image">
+                            <input type="text" id="card-background" class="form-control" placeholder="Background image URL" data-property="background_image">
                         </div>
                     </div>
                     <div class="form-group">