Add content-type header to curl examples in GitLab migration doc

What does this MR do and why?

Our API docs example to start a new migration does not work because it's missing the content-type header.

This MR adds the Content-Type: application/json header to the example in our docs:

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.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383750 (closed)