INTMD-428: doc update to "mongodbatlas_projects"

[Zuhairahmed]

Description

Doc bug fix - #867

Updated example to include "s" in mongodbatlas_projects

Please include a summary of the fix/feature/change, including any relevant motivation and context.

Link to any related issue(s):

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [x] Documentation fix/enhancement

Required Checklist:

• [x] I have signed the MongoDB CLA
• [ ] I have read the Terraform 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