Fix grammar in document store section (donnemartin#247)

Author: glusa8

README.md

@@ -972,7 +972,7 @@ A key-value store is the basis for more complex systems such as a document store
 
 A document store is centered around documents (XML, JSON, binary, etc), where a document stores all information for a given object.  Document stores provide APIs or a query language to query based on the internal structure of the document itself.  *Note, many key-value stores include features for working with a value's metadata, blurring the lines between these two storage types.*
 
-Based on the underlying implementation, documents are organized in either collections, tags, metadata, or directories.  Although documents can be organized or grouped together, documents may have fields that are completely different from each other.
+Based on the underlying implementation, documents are organized by collections, tags, metadata, or directories.  Although documents can be organized or grouped together, documents may have fields that are completely different from each other.
 
 Some document stores like [MongoDB](https://www.mongodb.com/mongodb-architecture) and [CouchDB](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/) also provide a SQL-like language to perform complex queries.  [DynamoDB](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/decandia07dynamo.pdf) supports both key-values and documents.