Update objective_function.md

[Junwenlaw]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes issue \

Please tag any associated issues here using the format Fixes #<issue number>. If there is no associated issue, please type No associated issue.

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?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• [ ] Example_Systems/SmallNewEngland/OneZone
• [ ] Example_Systems/SmallNewEngland/ThreeZones

Test Configuration:

• OS:
• Solver:
• Julia version:
• Solver version:

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have run tests with my code to avoid compatibility issues
• [ ] Any dependent changes have been merged and published in downstream modules