fix broken link of hydra paper

[seulbae-security]

---

Before sending a pull request, please review Contribution Guidelines:
https://github.com/google/syzkaller/blob/master/docs/contributing.md

---

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[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