Fix documentation example for editing of group_clusters

What does this MR do?

The edit group clusters API example had a wrong api_url attribute which should be part of platform_kubernetes_attributes. It was:

curl --header "Private-Token: <your_access_token>" "https://gitlab.example.com/api/v4/groups/26/clusters/24" \
-H "Content-Type:application/json" \
--request PUT --data '{"name":"new-cluster-name","domain":"new-domain.com","api_url":"https://new-api-url.com"}'

It should be similar to the add cluster API example https://docs.gitlab.com/ee/api/group_clusters.html#add-existing-cluster-to-group

curl --header "Private-Token: <your_access_token>" "https://gitlab.example.com/api/v4/groups/26/clusters/user" \
-H "Accept: application/json" \
-H "Content-Type:application/json" \
--request POST --data '{"name":"cluster-5", "platform_kubernetes_attributes":{"api_url":"https://35.111.51.20","token":"12345","ca_cert":"-----BEGIN CERTIFICATE-----\r\nhFiK1L61owwDQYJKoZIhvcNAQELBQAw\r\nLzEtMCsGA1UEAxMkZDA1YzQ1YjctNzdiMS00NDY0LThjNmEtMTQ0ZDJkZjM4ZDBj\r\nMB4XDTE4MTIyNzIwMDM1MVoXDTIzMTIyNjIxMDM1MVowLzEtMCsGA1UEAxMkZDA1\r\nYzQ1YjctNzdiMS00NDY0LThjNmEtMTQ0ZDJkZjM.......-----END CERTIFICATE-----"}}'

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