Add missing "of" before "duck typing".

This seems like a typo to me :
...similar to the concept      duck typing in dynamically...
...similar to the concept *of* duck typing in dynamically...

src/ch17-02-trait-objects.md

@@ -275,14 +275,15 @@ new type and draw it because `SelectBox` implements the `Draw` trait, which
 means it implements the `draw` method.
 
 This concept—of being concerned only with the messages a value responds to
-rather than the value’s concrete type—is similar to the concept *duck typing*
-in dynamically typed languages: if it walks like a duck and quacks like a duck,
-then it must be a duck! In the implementation of `run` on `Screen` in Listing
-17-5, `run` doesn’t need to know what the concrete type of each component is.
-It doesn’t check whether a component is an instance of a `Button` or a
-`SelectBox`, it just calls the `draw` method on the component. By specifying
-`Box<dyn Draw>` as the type of the values in the `components` vector, we’ve
-defined `Screen` to need values that we can call the `draw` method on.
+rather than the value’s concrete type—is similar to the concept of *duck
+typing* in dynamically typed languages: if it walks like a duck and quacks
+like a duck, then it must be a duck! In the implementation of `run` on `Screen`
+in Listing 17-5, `run` doesn’t need to know what the concrete type of each
+component is. It doesn’t check whether a component is an instance of a `Button`
+or a `SelectBox`, it just calls the `draw` method on the component. By
+specifying `Box<dyn Draw>` as the type of the values in the `components`
+vector, we’ve defined `Screen` to need values that we can call the `draw`
+method on.
 
 The advantage of using trait objects and Rust’s type system to write code
 similar to code using duck typing is that we never have to check whether a