Fix typo ('indrection' to 'indirection') (#218)

Author: shorii

idioms/coercion-arguments.md

@@ -14,7 +14,7 @@ Such as `&str` over `&String`, `&[T]` over `&Vec<T>`, or `&T` over `&Box<T>`.
 Using borrowed types you can avoid layers of indirection for those instances
 where the owned type already provides a layer of indirection. For instance, a
 `String` has a layer of indirection, so a `&String` will have two layers of
-indrection. We can avoid this by using `&str` instead, and letting `&String`
+indirection. We can avoid this by using `&str` instead, and letting `&String`
 coerce to a `&str` whenever the function is invoked.
 
 ## Example