docs: Fix introduction to refer to rules consistently
docs: Fix introduction to refer to rules consistently

The introduction referred to partial set rules as incremental rules
which is a bit misleading/confusing. This change just updates the
introduction to refer to partial rules instead of incremental rules.

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>

tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa