Fix script in Google Cloud IAM guided setup

What does this MR do and why?

Based on Guided setup for Google Cloud IAM integration - 3 (!146174 - merged).

Before:

export GL_PAT=<your_access_token>
curl --request GET
  --data 'google_cloud_project_id=<your_google_cloud_project_id>'
  --header "PRIVATE-TOKEN: $GL_PAT"
  --url "https://gitlab.com/api/v4/projects/<your_gitlab_project_id>/google_cloud/setup/wlif.sh"
| bash

After:

export GL_PAT=<your_access_token>
curl --request GET \
  --header "PRIVATE-TOKEN: $GL_PAT" \
  --data 'google_cloud_project_id=<your_google_cloud_project_id>' \
  --url "https://gitlab.com/api/v4/projects/<your_gitlab_project_id>/google_cloud/setup/wlif.sh" \
| bash

End lines with a backward slash. Without this the command will fail when copied and pasted into some shells 💥;

Also puts --header first to make it consistent with the script on the AR integration:

These all align with the documented guidelines and samples: https://docs.gitlab.com/ee/development/documentation/restful_api_styleguide.html#simple-curl-command

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Simulate a SAAS instance in local GDK

2. In rails console enable the required FF

   Feature.enable(:google_cloud_support_feature_flag)

3. Visit Settings > Integrations > Google Cloud IAM for any group or project

4. Click Guided setup