Fix pretty printer closing brace alignment

[ners]

Before this PR, prettyNix generates docs like this:

{
  foo = {
    bar = [
      a
      b
      c
      ]
    }
  }

With this PR, the closing braces are more correctly aligned with the opening ones:

{
  foo = {
    bar = [
      a
      b
      c
    ]
  }
}

[Anton-Latukha]

Thank you.