Fix warning about negative steps on Elixir 1.16

[davidwebster48]

When I run the formatter, I get this warning...

warning: negative steps are not supported in String.slice/2, pass 1..-2//1 instead
  (elixir 1.16.0) lib/string.ex:2368: String.slice/2
  (surface_formatter 0.7.5) lib/surface/formatter/phases/render.ex:133: Surface.Formatter.Phases.Render.render_node/2
  (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
  (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
  (surface_formatter 0.7.5) lib/surface/formatter/phases/render.ex:189: Surface.Formatter.Phases.Render.render_node/2
  (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2

This PR should fix it.

[paulstatezny]

Thank you!

[paulstatezny]

@msaraiva Good to merge this?