pgsql: doc: Improve pg_upgrade rsync examples to use clusterdir

doc: Improve pg_upgrade rsync examples to use clusterdir

Commit 9521ce4a7a1125385fb4de9689f345db594c516a from Sep 13, 2017 and
backpatched through 9.5 used rsync examples with datadir.  The reporter
has pointed out, and testing has verified, that clusterdir must be used,
so update the docs accordingly.

Reported-by: Don Seiler

Discussion: https://postgr.es/m/CAHJZqBD0u9dCERpYzK6BkRv=663AmH==DFJpVC=M4Xg_rq2=CQ@mail.gmail.com

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/de715414608846ce1ae44b79a39d61c48e25dce7

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)