Un-title case an overview item.

content/index.html

@@ -4,7 +4,7 @@ title: Home
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-      <h2><i class="fa fa-flask"></i> Data Model</h2>
+      <h2><i class="fa fa-flask"></i> Data model</h2>
       <p class="desc">Prometheus implements a highly dimensional data model. Time series are identified by a metric name and a set of key-value pairs.</p>
       <p><a class="btn btn-default" href="/docs/concepts/data_model/" role="button">View details &raquo;</a></p>
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