Fix word typo in documentation

docs-sources/usage/modifying.md

@@ -35,7 +35,7 @@ with the `Diff.ignore` instance.
 ## collection support
 
 Specify how objects within particular collection within particular diff instance should be matched.
-We distinguish free main types of collections:
+We distinguish three main types of collections:
 - seqLike collections where elements are indexed collections
 - setLike collections where elements aren't indexed
 - mapLike collections where elements(values) are indexed by some keys
@@ -54,4 +54,4 @@ implicit val diffOrg: Diff[Organization] = Diff.summon[Organization]
         // mapLike methods:
         .modify(_.peopleMap).matchByValue(_.age)
         .modify(_.peopleMap).matchByKey(_.weight)
-```
+```