Fix logo for documentation (#5337)

docs/templates/header.html

@@ -6,8 +6,8 @@
             class="md-header-nav__button md-logo">
           {% if theme_logo_icon|e %}
             <i class="md-icon">{{ theme_logo_icon }}</i>
-          {% elif logo %}
-              <img src="{{ pathto('_static/' ~ logo, 1) }}" height="26"
+          {% elif logo_url %}
+              <img src="{{ pathto(logo_url, 1) }}" height="26"
                     alt="{{ shorttitle|striptags|e }} logo">
           {% else %}
             &nbsp;