update hack/make-release-notes.sh with links to separated archives

[chriskim06]

Fixes #726

/assign @ahmetb
/assign @corneliusweig

[k8s-ci-robot]

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[chriskim06]

output from running this

$ TAG=v0.4.2 hack/make-release-notes.sh
Installation
------------
To install this release, refer to the instructions at https://github.com/kubernetes-sigs/krew/blob/v0.4.2/README.md.

Release Assets
--------------
Artifacts for this release can be downloaded from the following links.
It is recommended to follow [installation instructions](https://github.com/kubernetes-sigs/krew/blob/v0.4.2/README.md)
and not using these artifacts directly.

- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-darwin_amd64.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-darwin_amd64.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-darwin_arm64.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-darwin_arm64.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_amd64.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_amd64.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_arm.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_arm.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_arm64.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_arm64.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-windows_amd64.tar.gz
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-windows_amd64.tar.gz.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew.exe
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew.exe.sha256
- https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew.yaml

Thanks to our contributors for helping out with v0.4.2:
- Chris Kim
- Cornelius Weig
- Alexandre Garnier
- Ahmet Alp Balkan
- ferhat elmas
- Vincent
- Evgeny Shmarnev
- Chok Yip Lau
- Batuhan Apaydın

(krew v0.4.2 is tagged on Mon Oct 18 14:35:15 UTC 2021.)

<details>
<summary>Merged pull requests</summary>

Collecting pull request that were merged since the last release: v0.4.2 (2021-10-14 00:43:30 +0000 UTC)
* Adapt install doc for separated archives for each platform (#697) [#700](https://github.com/kubernetes-sigs/krew/pull/700)
* feat: make fmt.FPrintln instead of klog while printing error message [#719](https://github.com/kubernetes-sigs/krew/pull/719)
</details>

[ahmetb]

/lgtm