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

[anisa-hawes]

I am reinstating a link to a sample corpus referenced in the EN lesson working-with-batches-of-pdf-files which is now available again.

Closes #3659

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[anisa-hawes]

Hello Alex @hawc2.

This PR restores the link in working-with-batches-of-pdf-files which I adjusted yesterday. The sample corpus are back in place.