Revert incorrect emphasis changes back to italic

Per review feedback on #721:
- ifp_advanced.md: 'savings' - emphasis on grid type, not definition
- exchangeable.md: 'conditionally' - alternate description, not definition
- cass_koopmans_1.md: 'aggregation theory' - referencing theory, not defining it

These should remain italic for emphasis, not bold for definitions.

Author: mmcky

lectures/cass_koopmans_1.md

@@ -143,7 +143,7 @@ $$
 c(\omega) = c = C.
 $$
 
-This line of argument indicates the special **aggregation theory** that lies beneath outcomes in which a representative consumer
+This line of argument indicates the special *aggregation theory* that lies beneath outcomes in which a representative consumer
 consumes amount $C$.
 
 It appears often in aggregate economics.

lectures/exchangeable.md

@@ -47,7 +47,7 @@ search model.
 Among other things, this lecture discusses  connections between the statistical concepts of sequences of random variables that are
 
 - independently and identically distributed
-- exchangeable (also known as **conditionally** independently and identically distributed)
+- exchangeable (also known as *conditionally* independently and identically distributed)
 
 Understanding these concepts is essential for appreciating how Bayesian updating
 works.

lectures/ifp_advanced.md

@@ -264,7 +264,7 @@ assets is low.
 
 #### Finding Optimal Consumption
 
-The endogenous grid method (EGM) calls for us to take a grid of **savings**
+The endogenous grid method (EGM) calls for us to take a grid of *savings*
 values $s_i$, where each such $s$ is interpreted as $s = a -
 c$.