Clarify 'source dev.env' line at end of bootstrap.

In response to feedback that it looked like an error saying,
"you should have run 'source dev.env' before bootstrap."
In fact, it is saying bootstrap is done.

bootstrap.sh

@@ -160,4 +160,4 @@ echo "creating git pre-commit hooks"
 ln -sf $VTTOP/misc/git/pre-commit $VTTOP/.git/hooks/pre-commit
 
 echo
-echo "source dev.env in your shell before building."
+echo "bootstrap finished - run 'source dev.env' in your shell before building."