Clarify journal size based on filestore max sync

The docs had the recommended journal size based on the option
"filestore min sync interval" when it should have been
"filestore max sync interval".

While in there, fix a couple of typos -- multiple when it should
be multiply, and a missing word.  Change "Should at least twice"
to "Should be at least twice..."

Signed-off-by: Travis Rhoden <trhoden@gmail.com>