Why is my Boostrap-Card header not filling entire width of the card?

I am new to Bootstrap and Django. I am trying to style a card, but card header (background) does not fill entire width of the card. Here is an example:

Here is the code:

<div class="container">
  <div class="grid">
    <div class="row">
      {% for list, content in content.items %}

  <div class="card col-md-6 shadow-lg">
    <a href="{{user_name}}/{{list}}">
      <div class="card-header bg-secondary text-white">{{list}}</div>
      <div class="card-body">
        <ul>
          {% for row in content %}
          <li>{{row}}</li>
              {% endfor %}
            </ul>
          </div>
        </a>
      </div>
      {% endfor %}
    </div>
  </div>
</div>

You need to get rid of horizontal padding for the card. Add the px-0 class to the card div or p-0.

Regards.