Update flashcache-sa-guide.txt

There were two mentions of "writebehind" cache type.
From earlier declaration there are three types available: "Writethrough", "Writearound", "Writeback".
Probably "writebehind" means "Writearound".
replace those two mentions.
Merge pull request #122 from MaximF/patch-1

Update flashcache-sa-guide.txt