Update src/ch20-02-multithreaded.md

Co-Authored-By: Steve Klabnik <[email protected]>

src/ch20-02-multithreaded.md

@@ -364,7 +364,7 @@ impl ThreadPool {
 ```
 
 We still use the `()` after `FnOnce` because this `FnOnce` represents a closure
-that takes no parameters and returns an unit type `()`. Just like function
+that takes no parameters and returns the unit type `()`. Just like function
 definitions, the return type can be omitted from the signature, but even if we
 have no parameters, we still need the parentheses.