Fix documentation example syntax error

[Papajohn77]

In the documentation example Record selection from external database, the argument ["attached:selected.db"] was passed as a positional argument after the keyword argument condition, which caused a SyntaxError when running the code. To fix the issue, I added the key of the attach_databases parameter to the argument list. This ensures that the argument is correctly associated with the corresponding parameter in the function signature.

[dspinellis]

Excellent, thank you!

[dspinellis]

I'm closing this, because attach_databases is now part of the Crossref constructor. But please open a PR if you see another problem, or if you add something neat.