Fix typo in comment

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 5feebfed83fe463035ce64e6c2b10c2319842ff8
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2015-04-14T15:12:18Z
Releases: 9.0.20
Fix typo in comment

SLRU_SEGMENTS_PER_PAGE -> SLRU_PAGES_PER_SEGMENT

I introduced this ancient typo in subtrans.c and later propagated it to
multixact.c.  I fixed the latter in f741300c, but only back to 9.3;
backpatch to all supported branches for consistency.

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