Add missing 3 to page title

This was already present in the twitter meta tags

templates/base.j2

@@ -5,7 +5,7 @@
     <link rel="stylesheet" href="{{ site_root }}pygments.css">
     <link rel="stylesheet" href="{{ site_root }}pygments-dark.css">
     <link rel="shortcut icon" href="{{ site_root }}favicon.ico">
-    <title>{% block title %}{% endblock %} - mathlib docs</title>
+    <title>{% block title %}{% endblock %} - mathlib3 docs</title>
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta name="description" content="{% block metadesc %}The Lean 3 mathematical library, mathlib, is a community-driven effort to build a unified library of mathematics formalized in the Lean proof assistant.{% endblock %}" />