Fix plugin install location

[alexkursell]

The url for the plugin manifest should actually be under releases.

cc: @aledbf

[aledbf]

@alexkursell please check the release notes to install the plugin https://github.com/kubernetes/ingress-nginx/releases/tag/nginx-0.23.0 (--manifest only works with local files)

[alexkursell]

@alexkursell please check the release notes to install the plugin https://github.com/kubernetes/ingress-nginx/releases/tag/nginx-0.23.0 (--manifest only works with local files)

Oh, that makes sense! I'll just copy what's present on the release page then.

[aledbf]

/lgtm
/approve

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