Revise link to TopicsExplorer from working-with-batches-of-pdf-files

[anisa-hawes]

I'm revising a link to a sample corpus referenced in the EN lesson working-with-batches-of-pdf-files which is currently unavailable.

This sample corpus serves as an alternative learning option in the lesson, rather than the central case study.

Ref: #3659

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[anisa-hawes]

Hello Alex @hawc2. This PR adjusts a broken link in working-with-batches-of-pdf-files.

It isn't an essential component of the lesson - and I'm suggesting this as a temporary revision while I find out more.