Fix comment in btree_gist--1.8--1.9.sql

Hello Hackers,

I noticed that the comment at the top of btree_gist--1.8--1.9.sql says it is the 1.7--1.8 file. Here 
is a one-line patch to fix that.

A related question: In the v18 release we are adding two upgrade files: btree_gist--1.7--1.8.sql and 
btree_gist--1.8-1.9.sql. Why not collapse them into one? Technically we briefly had 
btree_gist--1.7--1.8.sql in 17devel, but it was reverted before release. Would you like a patch 
combining these files? Is it too late for that?

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com