etcupdate: Recommend building when bootstrapping

Since building is no longer the default, add -b to the bootstrapping
examples as they are likely to be run with a tree that hasn't been
built.

MFC after:	1 week
Fixes:		ddf6fad029 ("etcupdate: Make nobuild the default")
Reviewed by:	jhb
Differential Revision:	https://reviews.freebsd.org/D57643

usr.sbin/etcupdate/etcupdate.8

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 27, 2026
+.Dd June 18, 2026
 .Dt ETCUPDATE 8
 .Os
 .Sh NAME
@@ -699,7 +699,7 @@ before updating the source tree to start the next world upgrade cycle.
 First,
 generate a reference tree:
 .Pp
-.Dl "etcupdate extract"
+.Dl "etcupdate extract -b"
 .Pp
 Second,
 use the
@@ -725,7 +725,7 @@ The reference tree for
 .Nm
 can then be generated via:
 .Pp
-.Dl "etcupdate extract -s /path/to/tree"
+.Dl "etcupdate extract -b -s /path/to/tree"
 .Pp
 The
 .Cm diff