fixed typo (#175)

RobinKamps · samdark · commit 98a62e8afc28 · 2017-08-13T21:01:53.000+03:00
diff --git a/book/structure-collections.md b/book/structure-collections.md
@@ -1,7 +1,7 @@
 Implementing typed collections
 ==============================
 
-For stricter type hinting and iterfaces it could be useful to implement typed collections to put your models and other same-typed
+For stricter type hinting and interfaces it could be useful to implement typed collections to put your models and other same-typed
 classes into.
 
 As an example, we'll assume we have a `Post` class:
