Format: fix formatter bug on nesting begin/end

[makenowjust]

This file reproduces a formatter bug:

begin
  begin
  end
end

Error: couldn't format STDIN, please report a bug including the contents of it: https://github.com/crystal-lang/crystal/issues

expecting keyword end, not `(, `, at :2:3 (Exception)
  from Crystal::Formatter#check_keyword<Symbol>:Nil
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::Formatter>:(Bool | Nil)
  from Crystal::Formatter::format<String, Nil>:String
  from Crystal::Command#tool:(Bool | IO::FileDescriptor | Nil)
  from Crystal::Command#run:(Bool | Crystal::Compiler::Result | IO::FileDescriptor | Nil)
  from main

It is fixed by this PR.

[jhass]

Thank you!