Update _index.md

[bcrochet]

Add 'download/' to the OPERATOR_SDK_DL_URL

Description of the change:
The OPERATOR_SDK_DL_URL has changed. (Apparently by GitHub?). This adds 'download/' to the URL.

Motivation for the change:
The install docs are broken as they currently are.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[jmrodri]

/lgtm

[jmrodri]

Verified

Using the download link makes the binary accessible:
✔️ https://github.com/operator-framework/operator-sdk/releases/download/v1.6.1/operator-sdk_darwin_amd64
❌ https://github.com/operator-framework/operator-sdk/releases/v1.6.1/operator-sdk_darwin_amd64

Thanks @bcrochet

[estroz]

/cherry-pick v1.6.x

[openshift-cherrypick-robot]

@estroz: new pull request created: #4777

In response to this:

/cherry-pick v1.6.x

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[estroz]

/cherry-pick v1.5.x

[openshift-cherrypick-robot]

@estroz: #4770 failed to apply on top of branch "v1.5.x":

Applying: Update _index.md
Using index info to reconstruct a base tree...
M	website/content/en/docs/installation/_index.md
Falling back to patching base and 3-way merge...
Auto-merging website/content/en/docs/installation/_index.md
CONFLICT (content): Merge conflict in website/content/en/docs/installation/_index.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Update _index.md
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick v1.5.x

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.