pgsql: Improve terminology

Improve terminology

Use "prepared transaction" instead of "two-phrase transaction". This
is in line with c5d67881d343a507269bde124a49df19e0296157.

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8d985560fc1de6197323d4397d2a2bf762657463

Modified Files
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doc/src/sgml/ref/pg_recvlogical.sgml | 4 ++--
src/bin/pg_basebackup/pg_recvlogical.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)