Fix typo (donnemartin#29)

Author: ralphite

README.md

@@ -943,7 +943,7 @@ A key-value store generally allows for O(1) reads and writes and is often backed
 
 Key-value stores provide high performance and are often used for simple data models or for rapidly-changing data, such as an in-memory cache layer.  Since they offer only a limited set of operations, complexity is shifted to the application layer if additional operations are needed.
 
-A key-value store is the basis for more complex system systems such as a document store, and in some cases, a graph database.
+A key-value store is the basis for more complex systems such as a document store, and in some cases, a graph database.
 
 ##### Source(s) and further reading: key-value store