[doc] Fix broken snippet that had too many quotes
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_The documentation is not available anymore as the PR was closed or merged._ Spent the day familiarising myself with the huggingface line of products, and happened to run into some small issues here and there. Magically, I've found exactly one small issue in `transformers`, one in `accelerate` and now one in `datasets`, hah!

As for this PR, the issue seems solved according to the [new PR documentation](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4986/en/process#map):
![image](https://user-images.githubusercontent.com/37621491/190646405-6afa06fa-9eac-48f6-ab30-2677944fb7b6.png)

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

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