pgsql: Doc: Update description for parallel insert reloption.

Doc: Update description for parallel insert reloption.

Commit c8f78b6161 added a new reloption to enable inserts in parallel-mode
but forgot to update at one of the places about the same in docs. In
passing, fix a typo in the same commit.

Reported-by: Justin Pryzby
Author: Justin Pryzby
Reviewed-by: "Hou, Zhijie", Amit Kapila
Discussion: https://postgr.es/m/20210318025228.GE11765@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ed62d3737c1b823f796d974060b1d0295a3dd831

Modified Files
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doc/src/sgml/ref/alter_table.sgml  | 2 +-
doc/src/sgml/ref/create_table.sgml | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)