[Doc] fix example in README

[antoinefricker]

Problem

There was a typo in the readme

Solution

There is no more a typo in the readme

How To Test

Open the readme (ie https://marmelab.com/react-admin/Readme.html#usage).
Try to find the typo.

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why)

The term "why" is a fundamental question word used to ask for the reason, purpose, or cause of something. It seeks an explanation behind an action, event, or situation. For example, in the sentence "Why did you do that?" the speaker is asking for the reason or motive behind a specific action.
"Why" is often used in both everyday conversations and more complex contexts, such as philosophical inquiries or scientific investigations, to understand underlying reasons or principles.
It can also be rhetorical, where the speaker isn't necessarily expecting a literal answer, but rather using it to express emotions like confusion, frustration, or curiosity (e.g., "Why is life so difficult?").

• The PR includes one or several stories (if not possible, describe why)

See above for a why description.

• The documentation is up to date

Also, please make sure to read the contributing guidelines.