docs(framework) Fix typos on the FL Tutorial documentation

[dstripelis]

Issue

Minor fixes and paraphrases of the Flower documentation in the Federated Learning tutorial page: https://flower.ai/docs/framework/tutorial-series-what-is-federated-learning.html#What-is-Federated-Learning?

Description

• Fixing some syntactic and grammatical errors.
• Capitalizing all headers.
• Rephrasing some sentences to improve context.

Related issues/PRs

None

Proposal

Proposing grammatical and syntactical changes in the content of the What is Federated Learning? tutorial webpage to improve readability.

Explanation

• Changed Classic Machine Learning to Classical Machine Learning.
• Changed classic to classical wherever appropriate.
• Changed Central machine learning to centralized machine learning.
• Changed federated learning to Federated Learning in headers and when the term is introduced for the first time.
• Changed at zero to from zero.
• Added period at the end of the itemized list of where centralized machine learning is being used.
• Changed artificial intelligence to machine learning given that machine learning fits better in the current context when comparing centralized to federated learning.
• Added located when referring to a server.
• Capitalized headers Federated Evaluation, Federated Analytics, Final Remarks

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

None