Remove unused --mode arg from example config

[BrianJKoopman]

Description

See title.

Motivation and Context

Found this while reviewing a config change at site.

How Has This Been Tested?

Docs only change, not tested.

Types of changes

• 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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.