fix: update indexes-constraints page pgTable to the correct signature

[hill]

#433 updated these docs to the new signature in 0.36.0, where

...
  (table) => ({
    constraint: unique().on(table.something)
  })
)

becomes:

...,
  (table) => [
    unique().on(table.something)
  ]
)

but it left the PgTableExtraConfig in the array, which is incorrect and results in drizzle-kit not generating migrations. (i.e was updated to:

...,
  (table) => [{
    constraint: unique().on(table.something)
  }]
)

)

As per the mentioned diff, I believe it should be PgTableExtraConfigValue[] not PgTableExtraConfig[]

Let me know if I've misunderstood this and thanks for your time 🤗

Closes #456

[hill]

I believe this PR also closes #456