In here - https://svn0.us-west.freebsd.org/base/releng/11.0/README - at the bottom you will see this link - https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/synching.html for sync'ing your source code. The link is invalid, so the README should probably point to https://www.freebsd.org/doc/handbook/updating-src.html instead.
A commit references this bug: Author: gjb Date: Mon May 15 15:18:37 UTC 2017 New revision: 318294 URL: https://svnweb.freebsd.org/changeset/base/318294 Log: Correct the URL to instructions for updated system sources. MFC after: 3 days Submitted by: schaiba <at> gmail <dot> com PR: 219303 Sponsored by: The FreeBSD Foundation Changes: head/README
I've corrected this in head, and will MFC to stable branches in 3 days. I do not think this warrants an EN to releng branches, however.
A commit references this bug: Author: gjb Date: Thu May 18 12:27:41 UTC 2017 New revision: 318447 URL: https://svnweb.freebsd.org/changeset/base/318447 Log: MFC r318294: Correct the URL to instructions for updating system sources. PR: 219303 Sponsored by: The FreeBSD Foundation Changes: _U stable/10/ stable/10/README _U stable/11/ stable/11/README
MFC complete. Thank you for the report.