[doc] Fix broken snippet that had too many quotes
Hello!

### Pull request overview
* Fix broken snippet in https://huggingface.co/docs/datasets/main/en/process that has too many quotes

### Details
The snippet in question can be found here: https://huggingface.co/docs/datasets/main/en/process#map
This screenshot shows the issue, there is a quote too many, causing the snippet to be colored incorrectly:
![image](https://user-images.githubusercontent.com/37621491/190640627-f7587362-0e44-4464-a5d1-a0b98df6986f.png)

The change speaks for itself.

Thank you for the detailed documentation, by the way. 

- Tom Aarsen
