making this document consistent with the system-users-collection document

Signed-off-by: Sam Kleinman <samk@10gen.com>
suggested wording change

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update install-mongodb-on-os-x.txt

Update link to use new Brew website

Signed-off-by: Sam Kleinman <samk@10gen.com>