Source upgrade doc: etcupdate no longer needs -B

Note: We confusingly have the instructions to build the system in the
build manual, the UPDATING file, and the Makefile. These will get out
of sync and will be harmful when they do.

Fixes: ddf6fad0295a ("etcupdate: Make nobuild the default")
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D57644