doc/signup.md: update Writers before upspin-ui
doc/signup.md: update Writers before upspin-ui

Upspin-ui does not deal well with putting the servers
into config before the new user has write permission
on the servers. Perhaps that can be improved, but first
let's fix the doc.

Change-Id: I96da523ee722b9bd6464eae2bf73faa1ddeafcc1
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19401
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin