DOC: Corrected typo in warning on coerce
Low priority.

I encountered this warning message:

> "Setting an item of incompatible dtype is deprecated and will raise in a future **error** of pandas."

Which I believe should read:

> "Setting an item of incompatible dtype is deprecated and will raise **an error** in a future **version** of pandas."
OK merge for 2.2. Thanks @aaron-robeson-8451 