doc: Remove stray comma from list of psql options

doc: Remove stray comma from list of psql options

Back in 7.2 the list of options had short options and long options
on the same line separated by comma, but since 7.3 they are listed
separate lines. The comma on -X was left behind so fix by removing
and backpatching all the way.

Reported-by: [email protected]
Discussion: https://postgr.es/m/171267154345.684.7212826057932148541@wrigleys.postgresql.org
Backpatch-through: v12