curl examples in GitLab migration doc are missing content-type header

Problem

Our API docs example to start a new migration does not work. If example is used the error message is returned: "error":"configuration is missing even if configuration is missing.

The reason is a missing Content-Type: application/json header.

Proposal

Let's update our documentation and add the missing info:

curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/bulk_imports" \
+ --header "Content-Type: application/json" \
  --data '{
    "configuration": {
      "url": "http://gitlab.example/",
      "access_token": "access_token"
    },
    "entities": [
      {
        "source_full_path": "source/full/path",
        "source_type": "group_entity",
        "destination_slug": "destination_slug",
        "destination_namespace": "destination/namespace/path"
      }
    ]
  }'

We should probably include it in all of our docs.