contribution-stats: increase heading size of the tag name

templates/contribution-stats.html.j2

 {% for r in results %}
-    <h4>{{ r[0] }}</h4>
+    <h3>{{ r[0] }}</h3>
     {% for email, data in r[1] %}
         <strong>{{ contributors[email] }}{% if args.email %} &lt;{{ email }}&gt;{% endif %} ({{ data[0]}}):</strong><br />
         {% if not args.summary %}