Minor grammatical changes

[bigsweater]

Hi!

I was reading through the Lean guide and came across a couple small errors in the Groom Backlog section. Opened #8 without thinking too much about it---let me know if there are any other editorial steps y'all go through or anything of that sort! I hope I didn't presume.

And thanks for putting this info out in the open. It's really helpful.

vm