Update auth.md with small improvement to C# example

[thatstatsguy]

• better indication on how to reference the newly created playwright/.auth file relative to where tests are executing

[mxschmitt]

Isn't it even better to have the state.json inside the bin folder? Then the user never need to worry to add it to the gitignore file.

[thatstatsguy]

true - but what I'd do for this is remove the following part of the docs for c#

create a playwright/.auth folder and add it to the git ignore

since it's actually not needed (I assumed this had to be there for it to work). I assumed playwright would somehow automatically pick up that folder in the parent folder structure for the tests.