Merge pull request #2061 from tonytonyjan/patch-5

unit type value is also a value

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 doesn’t return a value. 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.