pgsql: doc: change "system" to "cluster" where appropriate

doc: change "system" to "cluster" where appropriate

Reported-by: Jeff Davis

Discussion: /messages/by-id/d040a1144e0127a49e335d1244a4de102a2a443b.camel@j-davis.com

Backpatch-through: master

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3a236fc9f39eb9eb39c1a66f7263590be3d2ed45

Modified Files
--------------
doc/src/sgml/config.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)