DOC: correct DataFrame.pivot docstring
The mention of panels that are created is not correct. You get a multi-index, I noticed

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14430?src=pr) is 85.25% (diff: 100%)

> Merging [#14430](https://codecov.io/gh/pandas-dev/pandas/pull/14430?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14430   diff @@
==========================================
  Files           140        140          
  Lines         50631      50631          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43167      43166     -1   
- Misses         7464       7465     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7cad3f1...3258159](https://codecov.io/gh/pandas-dev/pandas/compare/7cad3f16bccd1d4702ef9d038b1ee0db33b9bb94...325815904228575261ac6a77939d888e6391b87d?src=pr)
 lgtm.
