docs: Added using statement to document translation in README.md.

[benelori]

This PR contains a small fix, that includes the using statement to the document translation section of the README.

The reason I think this fix is needed is because without it the output file is empty after the translation.
Even if the fix is easy to make in the code, I think having a bit more precise example would benefit the documentation.

[JanEbbing]

Thanks for this fix! using outputFile = File.OpenWrite(outputDocumentPath); did not compile for me (on dotnet SDK 9.0.102), I needed to do using var outputFile = File.OpenWrite(outputDocumentPath);. I fixed it when fixing the merge conflicts.

I'll ask someone to approve this - do you want to rebase to remove the merge commit?