fix typo in readme.md

Author: duncandewhurst

README.md

@@ -33,7 +33,7 @@ Supporting documents with clarifications, or a full document containing answers
 
 Where the answers to a question are only available in attached documents, an `answer` value such as 'Consult section N of "%document name%" in the documents section' may be entered to allow analysts of the data to identify that an answer to this question has been provided.
 
-When a system allow a discussion format, where each answer can be followed by a further clarification question, the `threadID` property can be used to link together multiple entries in the `enquiries` array.
+When a system allows a discussion format, where each answer can be followed by a further clarification question, the `threadID` property can be used to link together multiple entries in the `enquiries` array.
 
 ## Usage guidance