update README.md

The link to the HashMap.java was broken. Directory changed from DataStructures/Hashmap/HashMap.java to DataStructures/Hashmap/Hashing/HashMap.java

Author: Mihir-Hindocha

README.md

@@ -185,5 +185,5 @@ Stacks|Trees|
 
 * [Bags](DataStructures/Bags/Bag.java)
 * [Buffer](DataStructures/Buffers/CircularBuffer.java)
-* [HashMap](DataStructures/HashMap/HashMap.java)
+* [HashMap](DataStructures/HashMap/Hashing/HashMap.java)
 * [Matrix](DataStructures/Matrix/Matrix.java)