Fix layout of logo.

Having <h1> inside <p> led some browsers to believe that the HTML code was
ill-formed. Thus, they inserted </p> before <h1>, causing the logo to be
pushed away from the logo name.

alabaster/about.html

@@ -1,14 +1,14 @@
 {% if theme_logo %}
-<p class="logo">
+<h1 class="logo">
   <a href="{{ pathto(master_doc) }}">
     <img class="logo" src="{{ pathto('_static/' ~ theme_logo, 1) }}" alt="Logo" />
     {% if theme_logo_name|lower == 'true' %}
-    <h1 class="logo logo-name">{{ project }}</h1>
+    <span class="logo logo-name">{{ project }}</span>
     {% elif theme_logo_name|lower != 'false' %}
-    <h1 class="logo logo-name">{{ theme_logo_name }}</h1>
+    <span class="logo logo-name">{{ theme_logo_name }}</span>
     {% endif %}
   </a>
-</p>
+</h1>
 {% else %}
 <h1 class="logo"><a href="{{ pathto(master_doc) }}">{{ project }}</a></h1>
 {% endif %}