Rename Master-Slave in Systems.md

[darylldoyle]

Is your enhancement related to a problem? Please describe.
As per the "Inclusivity in Code" document, the master/slave database naming convention is to be avoided and primary/replica used in its place. As such, the Systems.md document should be updated to reflect this change in terminology.

Describe the solution you'd like
References to Master and Slave should be updated to use Primary and Replica respectively.