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
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/350f4f96bb9b7188df7fd4f755fe94562436853b

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