DOC: Fix wording/grammar for rolling's win_type argument.
I don't know the exact intended phrasing here, but I think the writer might have meant "Provide"?

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13376?src=pr) is **84.23%**

> Merging [#13376](https://codecov.io/gh/pydata/pandas/pull/13376?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13376   diff @@
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  Methods           0          0          
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  Branches          0          0          
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  Hits          42740      42740          
  Misses         7999       7999          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [b722222...ec0c88e](https://codecov.io/gh/pydata/pandas/compare/b722222f5ea760a3f3df4d063309949eb4956674...ec0c88e812ea607518b133d269e8082566c53791)
 thanks @mikegraham 

yeah prob could flesh that out a bit
