Fix GIST readme on LSN vs NSN

The GiST README says:

> If the F_FOLLOW_RIGHT flag is not set, a scan compares the NSN on the
> child and the LSN it saw in the parent. If NSN < LSN, the scan looked
> at the parent page before the downlink was inserted, so it should
> follow the rightlink. Otherwise the scan saw the downlink in the
> parent page, and will/did follow that as usual.

While the code does this (in gistget.c):

> 	if (!XLogRecPtrIsInvalid(pageItem->data.parentlsn) &&
> 		(GistFollowRight(page) ||
> 		 pageItem->data.parentlsn < GistPageGetNSN(page)) &&
> 		opaque->rightlink != InvalidBlockNumber /* sanity check */ )
> 	{
> 		/* There was a page split, follow right link to add pages */

Note the comparison on LSN and NSN. The code seems correct, but the 
README got it backwards.

The narrow fix would be to change the "NSN < LSN" to "LSN < NSN" in the 
README. But I propose the attached patch to reword the sentence a little 
more.

-- 
Heikki Linnakangas
Neon (https://neon.tech)