Quick one to fix a formatting issue (spotted in #395 (review)) due to a (valid) change that went in as part of #384 but which has been partially reverted, likely due to a merge conflict resolution mistake somewhere along the line.

Before this PR for e.g. d.ismasked() on some cf.Data:

DeprecationError: Data attribute 'ismasked' has been deprecated at version TODODASK and will be removed at version  and will be removed at version 5.0.0. Use the 'is_masked' attribute instead

And after this PR, the obvious fix is made:

DeprecationError: Data attribute 'ismasked' has been deprecated at version TODODASK and will be removed at version 5.0.0. Use the 'is_masked' attribute instead

No review requested as this is a trivial fix.