Update HOME_EXAM.md

HOME_EXAM.md

@@ -304,7 +304,7 @@ Other boolean expressions are
 
 ### Let expression
 ```math
-\frac{<v, σ>⇓<\text{let } v:=e, σ>⇓σ'}{<\text{let } v := e, σ> ⇓ σ[v := e]}
+\frac{<v, σ>⇓<\text{let } v:t, σ> ⇓ σ'}{<\text{let } v : t, σ> ⇓ σ[v : t]}
 ```
 
 ```math