Fix emphasis vs definitions in ak2.md

Changes per #721:
- numeraire

Term changed from italic to bold as it is a definition per style guide.

Author: mmcky

lectures/ak2.md

@@ -209,7 +209,7 @@ Units of the rental rates are:
 * for $r_t$,  output at time $t$  per unit of capital at time $t$ 
 
 
-We take output at time $t$ as *numeraire*, so the price of output at time $t$ is one.
+We take output at time $t$ as **numeraire**, so the price of output at time $t$ is one.
 
 The firm's profits at time $t$ are