fix curl command in doc/api/groups.md

ruuu-05requested to merge
gitlab-community/gitlab-org/gitlab:fix-curl-doc/api/groups.md into master

What does this MR do and why?

Updates the cURL example in doc/api/groups.md to follow the API documentation cURL formatting guidelines.

The command now:

  • explicitly specifies --request GET
  • places each parameter on a separate line
  • explicitly specifies --url

What are the relevant issue numbers?

#509818

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