Correct presenter and TTY documentation

[OskarEichler]

Summary

Correct two README statements to match the runtime behavior:

• Enabled presenter formatting methods return a new Rainbow::Presenter, which is why calls can be chained; they do not return self.
• Automatic terminal detection checks STDOUT and STDERR, not STDIN and STDERR.

Verification

Confirmed against Presenter#wrap_with_sgr and the initialization conditions in lib/rainbow.rb. git diff --check passes. Documentation only; no runtime, dependency or test changes.

Compatibility

No breaking change.