document mistake

[蔣彥亭]

http://www.freebsd.org/doc/handbook/firewalls-pf.html
In 30.4.6.6. Tables Make Life Easier
demo code
table <clients> persist file /etc/clients the file path should be quote.
table <clients> persist file "/etc/clients" or it would show a syntax error
when pf try to load the configuration file.
Thanks!

My OS: FreeBSD 9.0 i386