Update README.md

Co-authored-by: Julia Damerow <[email protected]>

README.md

@@ -24,7 +24,7 @@ Read [Contributors](CONTRIBUTORS.md) for detailed contribution information.
 
 ## Example Usage
 
-Often humanities and cultural data includes imprecise or uncertain temporal information. We want to store that information but also work with it in a structured way, not just treat it as text for display.  Different projects may need to work with or convert between different date formats or even different calendars.
+Often humanities and cultural data include imprecise or uncertain temporal information. We want to store that information but also work with it in a structured way, not just treat it as text for display.  Different projects may need to work with or convert between different date formats or even different calendars.
 
 An `undate.Undate` is analogous to python's builtin `datetime.date` object, but with support for varying degrees of precision and unknown information.  You can initialize an undate with either strings or numbers for whichever parts of the date are known or partially known.  An `Undate` can take an optional label.
 ```python