Fix to yaml examples 0115

[maxim-uvarov]

Fix two to yaml examples in the 0.115.0 release notes
Both snippets do not match what Nushell 0.115.0 actually does.

--non-roundtrip takes a string, so a bare null is parsed as the
null literal and the line fails to parse:

Error: nu::parser::parse_mismatch_with_full_string_msg
  x Parse mismatch: expected string.

Quoting it makes the example run and print null as documented.

The --indent 4 --quote double example showed output with 2-space
indentation and single quotes. The real output on 0.115.0 is:

outer:
    inner: "value"

which is also what the two flags are there to demonstrate.