Fix typo - errors5.rs

Minor typo correction in exercise instructions

Author: cassian-goode

exercises/13_error_handling/errors5.rs

@@ -6,7 +6,7 @@
 //
 // In short, this particular use case for boxes is for when you want to own a
 // value and you care only that it is a type which implements a particular
-// trait. To do so, The `Box` is declared as of type `Box<dyn Trait>` where
+// trait. To do so, the `Box` is declared as of type `Box<dyn Trait>` where
 // `Trait` is the trait the compiler looks for on any value used in that
 // context. For this exercise, that context is the potential errors which
 // can be returned in a `Result`.