Fix docker hub url for valkey

[rafael1mc]

Noticed that the docker hub url for valkey was leading to a 404 page, so I fixed it.

I looked for others, and this seems to be the only app not having either /r or /_.

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First of all, thank you for your contribution! 😄

Please note that this repo is mostly for popular apps with thousands of stars and tens of thousands of downloads. If you'd like to add a less popular app, you can always create your own 3rd party repo and add your app there.

☑️ Self Check before Merge

• I have tested the template using the method described in README.md thoroughly

[githubsaturn]

Thanks!