doc: fix grammar in doc/server_setup.md
doc: fix grammar in doc/server_setup.md

Respond to Andrew's comment on CL 7887.

Change-Id: I220fed7bbec0aa5a713770a2e4fe65f5f7d38c50
Reviewed-on: https://upspin-review.googlesource.com/7888
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin