doc: fix typo in doc/security.md
doc: fix typo in doc/security.md

Change-Id: I06907c7559fdae5d15751b22e992cbcc11780bdb
Reviewed-on: https://upspin-review.googlesource.com/15120
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin