DOC: fix an example which raises an Error in whatsnew/v0.24.0.rst
Fix an example in doc/source/whatsnew/v0.24.0.rst, which raises an Error (see https://pandas.pydata.org/docs/dev/whatsnew/v0.24.0.html#integer-addition-subtraction-with-datetimes-and-timedeltas-is-deprecated)
@MarcoGorelli, could you please take a look at this PR?