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:	ddf6fad029 ("etcupdate: Make nobuild the default")
Reviewed by:		jhb
Differential Revision:	https://reviews.freebsd.org/D57644

Makefile

@@ -85,7 +85,7 @@
 #  5.  `reboot'        (in single user mode: boot -s from the loader prompt).
 #  6.  `etcupdate -p'
 #  7.  `make installworld'
-#  8.  `etcupdate -B'
+#  8.  `etcupdate'
 #  9.  `make delete-old'
 # 10.  `reboot'
 # 11.  `make delete-old-libs' (in case no 3rd party program uses them anymore)