fix broken link of hydra paper

[seulbae-security]

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[google-cla[bot]]

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[a-nogikh]

Thanks!

Please note that you need to sign the CLA (as the comment above suggests) and you'd need to change the commit title to something like doc: fix broken link of hydra paper (that's why the ci/aux test failed).

https://github.com/google/syzkaller/blob/master/docs/contributing.md#commits