update documentation with the new OS update check interval

It was increased from 4 hours to 6 hours.

static/usage.html

@@ -396,7 +396,7 @@
                 <h2><a href="#updates">Updates</a></h2>
 
                 <p>The update system implements automatic background updates. It checks for updates
-                approximately once every four hours when there's network connectivity and then
+                approximately once every six hours when there's network connectivity and then
                 downloads and installs updates in the background. It will pick up where it left off if
                 downloads are interrupted, so you don't need to worry about interrupting it.
                 Similarly, interrupting the installation isn't a risk because updates are installed to