DOC: fix incorrect example in unstack docstring (GH14206)
- [x] closes #14206 

The following PR closes #14206 and corrects the Docs as suggested in the issue thread.
@jorisvandenbossche Kindly review.

I don't think you actually changed the output? (apart from the floating notation, which is OK). It is the output of `df.unstack()` that was wrong (the 2 and 3 are swapped), and which should be corrected.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14211?src=pr) is 85.24% (diff: 100%)

> Merging [#14211](https://codecov.io/gh/pydata/pandas/pull/14211?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14211   diff @@
==========================================
  Files           140        140          
  Lines         50563      50566     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43103      43103          
- Misses         7460       7463     +3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [461e0e9...99c2f0b](https://codecov.io/gh/pydata/pandas/compare/461e0e9fba67b1f5cbe6ac5bd1db0272042777ee...99c2f0b6adf79571d235a1db48cb57b3884178d8?src=pr)
 @jorisvandenbossche : Oops, my bad. I corrected the mistake now.
 @sudk1896 Thanks for the fix! 
(small note: you had a small trailing whitespace error which was captured by our linter (I corrected it), but I would recommend setting your editor to automatically remove trailing whitespace)
 @jorisvandenbossche: Will do from next time. Thanks.
