Fix a few typos in brin_minmax_multi.c

Hi,

During a recent cleanup of brin_minmax_multi.c I noticed a few typos.
I've attached a patch to fix these.

I originally buried this in [1], but think it's likely better to have
a proper thread for it.

The patch does change some comments which reference parameter or
variable names. I hope that I've not misunderstood something.  It
would be good if Tomas could have a look over it just in case I have.

David

[1] https://www.postgresql.org/message-id/CAApHDvqPKwbUn8Z74BZrDmLoZzcP4s%2BLZTxNuUi3P3OxbieT1Q%40mail.gmail.com