mongodbatlas_privatelink_endpoint_service data source doc bug fix

[Zuhairahmed]

Description

Please include a summary of the fix/feature/change, including any relevant motivation and context.
Missing GCP as valid input per Atlas Admin API Docs: https://www.mongodb.com/docs/atlas/reference/api-resources-spec/v2/#tag/Private-Endpoint-Services/operation/createPrivateEndpoint

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[andreaangiolillo]

LGTM