update indentation.md: fix flags - prefix

[unkarjedy]

Also, looks like -language:Scala2 doesn't disable indentation-based-syntax.
For example, this is compiled fine and prints 123123123:

@main
def main1(): Unit = {
  def foo() =
    println(1)
    println(2)
    println(3)

  foo()
  foo()
  foo()
}

[som-snytt]

Do you wish to add the clarification about -old-syntax? I already joked that it should be -mature-syntax.

[unkarjedy]

I initially wanted to add the clarification for -old-syntax in
https://scalacenter.github.io/scala-3-migration-guide/docs/compiler-options/new-compiler-options.html
(Currently, the description is quite poor. It doesn't mention all other new control syntax, and bracelets syntax)

But I decided not to do that because I can't tell that my clarification will be entirely valid.
That's taking into account that inconsistency between spec, impl (and also expectations) is quite frequent in Scala.
To completely understand what the flag is intended for you have to dive into the compiler and see all its usages.
So the best would be if the author would review it and fix the description.

[unkarjedy]

Anyway, if a valid description existed, I think it would be nice to collect complete descriptions in a single table
(E.g. https://scalacenter.github.io/scala-3-migration-guide/docs/compiler-options/new-compiler-options.html ?)
and add cross-references between the the table and mentions in feature descriptions at http://dotty.epfl.ch/docs/