pgsql: Fix outdated --help message for postgres -f

Fix outdated --help message for postgres -f

This option switch supports a total of 8 values, as told by
set_plan_disabling_options() and the documentation, but this was not
reflected in the output generated by --help.

Author: Junwang Zhao
Discussion: /messages/by-id/CAEG8a3+pT3cWzyjzKs184L1XMNm8NDnoJLiSjAYSO7XqpRh_vA@mail.gmail.com
Backpatch-through: 10

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a5d2337005820a2227d2d9970cdd5a7ac67bfc75

Modified Files
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src/backend/main/main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)