Docs: fix json response

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Update this page : https://docs.gitlab.com/ee/api/namespaces.html

Add the name field in the json example response for GET /namespaces

Example : https://gitlab.com/api/v4/namespaces

[{ "id": 1906873, "name": "testNamespace", "path": "test-namespace", "kind": "group", "full_path": "test-namespace", "parent_id": null, "members_count_with_descendants": 1, "plan": null }]