doc: add upspin-ui go get instructions
doc: add upspin-ui go get instructions

The signup documentation doesn't mention that upspin-ui is not in the standard
upspin package it instructs to go get. Also fixes a formatting error.

Change-Id: I1501030d43db65f1932d57d3c901635aa75befee
Reviewed-on: https://upspin-review.googlesource.com/17900
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin