pgsql: Fix a missing article in the documentation

Fix a missing article in the documentation

Per complaint from Grant Gryczan.

It's a very old typo; backpatch all the way back.

Author: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/172179789219.915368.16590585529628354757@wrigleys.postgresql.org

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0cc57dca298c86403b6d6bb647f99d542f9d3dca

Modified Files
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doc/src/sgml/parallel.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)