Fix curl example request method of Namespace list API

What does this MR do?

Fix a typo in documentation. The Namespace list API should be GET method, not a PUT method.

The grape routes suggests this is a GET method:

$ bin/rails grape:routes | grep "namespaces list" 
GET /api/:version/namespaces(.:format) - Get a namespaces list

Another document page(https://docs.gitlab.com/ee/api/namespaces.html#list-namespaces) also suggests this is a GET method.

Also confirmed on my local development environment:

$ curl --request GET --header "PRIVATE-TOKEN: N6-5z8kX1Cf6kXaoxzNZ" "http://localhost:3000/api/v4/namespaces?per_page=1"
[{"id":1,"name":"Administrator Lastname","path":"root","kind":"user","full_path":"root","parent_id":null,"avatar_url":"https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon","web_url":"http://192.168.0.64:3000/root","shared_runners_minutes_limit":10000,"extra_shared_runners_minutes_limit":null,"additional_purchased_storage_size":0,"additional_purchased_storage_ends_on":null,"billable_members_count":1,"seats_in_use":1,"max_seats_used":1,"plan":"premium","trial_ends_on":null,"trial":false}]


$ curl --request PUT --header "PRIVATE-TOKEN: N6-5z8kX1Cf6kXaoxzNZ" "http://localhost:3000/api/v4/namespaces?per_page=1"
{"error":"404 Not Found"}

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