README.md: Fix spelling in ARNEIS acronym definition

[gmacario]

📑 Description

Change to reflect the decision made in the ARNEIS video (#151).

Blocks #282

✅ Checks

• [x] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [x] I have updated the documentation as required
• [x] All the tests have passed

ℹ Additional Information

Credits to @davmacario for pointing us out the spelling mistake. For sure he nows American English much better than the ARNEIS core team.

[gmacario]

Thanks @gteti, I assume @alv67 will also agree. Merging PR

[alv67]

@gmacario uou are right: I agree.