Updated strapline in docs (and fixed mis-matched HTML tags in template)

Author: tturocy

doc/_templates/layout.html

@@ -16,7 +16,7 @@
     <a href="https://www.gambit-project.org/"><img src="{{
     pathto("_static/Gambit_logo_landscape_green_font_no_outline.png", 1) }}" border="0" alt="Chadwick" style="width: 12%;"/></a>
 
-    <h4 style="color: #1a9e7e; margin: 0 auto; text-align: center; width: 100%;">Software tools for game theory.</h3>
+    <h4 style="color: #1a9e7e; margin: 0 auto; text-align: center; width: 100%;">The package for computation in game theory</h4>
 </div>
 {{ super() }}
 {% endblock %}