Rename Grok to xAI

[clementbiron]

When this service was added, the name was incorrect. It was then corrected from Grok to xAI in OpenTermsArchive/genai-contrib-declarations#134.

We will therefore need to take into account any changes in the history of this service.

[MattiSG]

Done in dataset through OpenTermsArchive/engine#1076. Not visible in versions.