Jonas Jenwald 0083f63dda Fix a typo in the "file" entry for issue3694_reduced in test_manifest.json
When generating new references locally on Windows, after PR 6724, I get the following output:
```
WARNING: Unable to open file for reading "Error: ENOENT, open 'c:\Users\Jonas\Git\pdfjs\test\pdfs\issue_3694_reduced.pdf'".
Unable to verify the checksum for the files that are used for testing.
Please re-download the files, or adjust the MD5 checksum in the manifest for the files listed above.

```

Compared to the name of the file (`issue3694_reduced.pdf`), you see that the manifest entry has a superfluous underscore in the "file" entry.
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