Change extension -> evaluation in docs
Extension seems to be incorrect as this is referring to the valuation model
Closes
@@ -19,7 +19,7 @@ user-provided inputs to a rule, but also all of the tools and libraries required
to execute the actions (called _implicit inputs_).
Before creating or modifying any rule, make sure you are familiar with the
-[extensibility model](concepts.md) (understand the three phases and the
+[evaluation model](concepts.md) (understand the three phases of execution and the
differences between macros and rules).
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