Fix typo in README about invertability

Author: laughinghan

README.md

@@ -14,7 +14,7 @@ The JSON OT type supports the following operations:
 
 JSON0 is an *invertable* type - which is to say, all operations have an inverse
 operation which will undo the original op. As such, all operations which delete
-content add the content to be deleted inline in the operation.
+content have the content to be deleted inline in the operation.
 
 But its not perfect - here's a list of things it *cannot* do: