Update terminology in usage statistics page
<p>Jenkins reports the following general usage statistics:</p>
<ul>
<li>Your Jenkins version</li>
- <li>For your master and each agent, the OS type and number of executors</li>
+ <li>For your controller and each agent, the OS type and number of executors</li>
<li>Installed plugins and their versions</li>
<li>Number of items (like jobs) of each item type</li>
</ul>
