Add missing link to Update section to Guide

FreeNAS Manual version 11.2-U3

Section 2.5.4:  https://www.ixsystems.com/documentation/freenas/11.2/install.html#upgrading-from-the-web-interface 

The section references going to "System" --> "Update", and provides no other information. However section 6.11 goes into great detail regarding the webui upgrade process. Since the remainder of section 2.5 is in regards to installing, and upgrading via the terminal, 2.5.4 should simply link over to 6.11