Docs: Specify '--request PUT' in cURL call example

What does this MR do?

The "Rebase a merge request" REST endpoint expects a PUT call:

PUT /projects/:id/merge_requests/:merge_request_iid/rebase

The cURL example in the documentation does a normal GET call:

curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/76/merge_requests/1/rebase

To make it a PUT call, the --request PUT option should be added:

curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/76/merge_requests/1/rebase

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