90-build-vm.mk: better error reference

In this case it's rather worth it to examine build.log
than read documentation again (as vm.txt should have been
read or at least skimmed through to get sudo setup ready,
and the problem might be either an environment one or a bug).

features.in/build-vm/lib/90-build-vm.mk

@@ -25,7 +25,7 @@ prepare-image: check-sudo
 	fi; \
 	if ! sudo $(TOPDIR)/bin/tar2fs \
 		"$(VM_TARBALL)" "$(VM_RAWDISK)"  $(VM_SIZE) $(VM_FSTYPE); then \
-		echo "** error: sudo tar2fs failed, see also doc/vm.txt" >&2; \
+		echo "** error: sudo tar2fs failed, see build log" >&2; \
 		exit 1; \
 	fi