Merge pull request #2709 from drybalka/patch-1

Update concrete-immutable-collection-classes.md

Author: Philippus

_overviews/collections-2.13/concrete-immutable-collection-classes.md

@@ -232,7 +232,7 @@ Ranges are represented in constant space, because they can be defined by just th
 
 Hash tries are a standard way to implement immutable sets and maps efficiently. [Compressed Hash-Array Mapped Prefix-trees](https://github.com/msteindorfer/oopsla15-artifact/) are a design for hash tries on the JVM which improves locality and makes sure the trees remain in a canonical and compact representation. They are supported by class [immutable.HashMap](https://www.scala-lang.org/api/{{ site.scala-version }}/scala/collection/immutable/HashMap.html). Their representation is similar to vectors in that they are also trees where every node has 32 elements or 32 subtrees. But the selection of these keys is now done based on hash code. For instance, to find a given key in a map, one first takes the hash code of the key. Then, the lowest 5 bits of the hash code are used to select the first subtree, followed by the next 5 bits and so on. The selection stops once all elements stored in a node have hash codes that differ from each other in the bits that are selected up to this level.
 
-Hash tries strike a nice balance between reasonably fast lookups and reasonably efficient functional insertions (`+`) and deletions (`-`). That's why they underly Scala's default implementations of immutable maps and sets. In fact, Scala has a further optimization for immutable sets and maps that contain less than five elements. Sets and maps with one to four elements are stored as single objects that just contain the elements (or key/value pairs in the case of a map) as fields. The empty immutable set and the empty immutable map is in each case a single object - there's no need to duplicate storage for those because an empty immutable set or map will always stay empty.
+Hash tries strike a nice balance between reasonably fast lookups and reasonably efficient functional insertions (`+`) and deletions (`-`). That's why they underlie Scala's default implementations of immutable maps and sets. In fact, Scala has a further optimization for immutable sets and maps that contain less than five elements. Sets and maps with one to four elements are stored as single objects that just contain the elements (or key/value pairs in the case of a map) as fields. The empty immutable set and the empty immutable map is in each case a single object - there's no need to duplicate storage for those because an empty immutable set or map will always stay empty.
 
 ## Red-Black Trees