Update doc/flags.md

Co-authored-by: Mathias Hauser <[email protected]>

Author: dcherian

doc/flags.md

@@ -62,7 +62,7 @@ da.cf.is_flag_variable
 
 ## GroupBy
 
-Flag variables, such as that above, are naturally used for GroupBy operations.
+Flag variables, such as the one above, are naturally used for GroupBy operations.
 cf-xarray provides a `FlagGrouper` that understands the `flag_meanings` and `flag_values` attributes.
 
 Let's load an example dataset where the `flag_var` array has the needed attributes.