fix pluralize for invoice header

[apoorv1316]

Summary

• FIx pluralization for selected invoices.

Preview

Type of change

Please delete options that are not relevant.

• 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

How Has This Been Tested?

Checklist:

• I have manually tested all workflows
• I have performed a self-review of my own code
• I have added automated tests for my code

[github-actions]

Current Code Coverage Percent of this PR:

97.79 %

Files having coverage below 100%

Impacted Files	Coverage
/app/controllers/clients_controller.rb	94.44 %
/app/models/project.rb	90.32 %
/app/controllers/users/omniauth_callbacks_controller.rb	25.0 %
/app/controllers/internal_api/v1/timesheet_entry/bulk_action_controller.rb	50.0 %