Fix typo (change 'n' to 'k' in get_dummies documentation)
Just changing an `n` to a `k`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14153?src=pr) is 85.25% (diff: 100%)

> Merging [#14153](https://codecov.io/gh/pydata/pandas/pull/14153?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14153   diff @@
==========================================
  Files           139        139          
  Lines         50502      50502          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43058      43058          
  Misses         7444       7444          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [900ae6b...d725192](https://codecov.io/gh/pydata/pandas/compare/900ae6bfd4a940f7a165ba2635eb590952f0ce84...d72519257802dc7a05061b1deae98ec314dfda27?src=pr)
 Thanks!
