replace slave label with replica

[stephanos]

Description

One of the example is using the antiquated word "slave" for a database replica. I updated the language and tested the change.

Motivation and Context

If possible, I'd like to live in a world where we avoid that word.

How Has This Been Tested?

I've tested the example on my local machine and it seems to work perfectly fine:

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 change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md

[dead-claudia]

I'll accept it as databases are moving to that terminology anyways (and I would prefer to stay consistent with their idioms above all else).