Correction to comment of brin_range_deserialize

Hi Hackers,

I found an issue in the comments of the
src/backend/access/brin/brin_minmax_multi.c file.

The header comment of function brin_range_deserialize currently reads:
"Serialize the in-memory representation into a compact varlena value."

which looks like a copy/paste error, where "Serialize" should be
"Deserialize".

I'm posting this patch to fix the comment.

Regards,
ji xu