DOC: fix comment on previous versions cythonmagic
Small thing I just noticed in the docs (the note on the other version was not updated when the example was changed from cythonmagic -> Cython)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13343?src=pr) is **84.23%**

> Merging [#13343](https://codecov.io/gh/pydata/pandas/pull/13343?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13343   diff @@
==========================================
  Files           138        138          
  Lines         50713      50713          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42715      42714     -1   
- Misses         7998       7999     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [fcd73ad...902352c](https://codecov.io/gh/pydata/pandas/compare/fcd73ad2e7482414b61d47056c6c9c220b11702c...902352c659276e2ea9c05e5af88b637cbf6918ee)
