Example added for database user scope

[nikhil-mongo]

Description

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
• Documentation fix/enhancement

Required Checklist:

• 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 requirments
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

This is the addition to the examples directory for the database user scope.

[themantissa]

LGTM - see comments for nits/good practice advice for future PRs. If you can adjust the newlines quickly I'll give a bit of time before I merge. Appreciate your quick work on this!