DOC: fixup formula for exponentially weighted mean with input vector …
…of times? #40327

- [x] closes #40327 
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Thanks, looks good. Could you render the doc and post a picture to confirm the change worked? Old version and new version:

![image](https://user-images.githubusercontent.com/37313715/110594968-13358a80-8143-11eb-99ee-4523d6dbb436.png)
 Thanks @mschmookler !