MINOR: [C#] Fixing example to use WriteEndAsync instead of WriteFooterAsync

[royalstream]

WriteFooterAsync is private, so the example doesn't compile.
This method was probably public in an earlier version of the library.
WriteEndAsync seems to be the proper replacement.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on JIRA? https://issues.apache.org/jira/browse/ARROW

Opening JIRAs ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename pull request title in the following format?

ARROW-${JIRA_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches

[eerhardt]

Thanks!