expansions.asciidoc: Add an example of file expansion.

Properly speaking, all expansions are of the form "%abc{}" where "abc"
is the expansion type. All expansion types are listed in the documentation, as
they should be. However, when searching through the documentation, a keyword
like "file" is likely to hit a lot of false-positives, so a smart user is likely
to search for the more-unique "%file" instead. Unfortunately, unlike every other
expansion-type, file-expansions did not include an example, so a search for
"%file" resulted in no hits, potentially leading people to believe it was
undocumented.

Now there's an example, so people searching for "%file" will find what they're
looking for.

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