add a serial (Oxford) comma

[RichMorin]

On a vaguely related note, I can't figure out how/where to submit changes to these pages:

• https://www.erlang.org/doc/man/xref.html
• https://www.erlang.org/doc/apps/tools/xref_chapter.html

Help?

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[garazdawi]

Thanks! The documentation for xref is located here. You can read about how to write and build documentation here.