[docs] [issue18705] Fix arround 100 typos/spelling mistakes

[Févry Thibault]

Févry Thibault added the comment:

> I don't think there's a strict policy about using American spellings in the source;  I think spellings like 'behaviour' and 'grey' should be left alone.

I didn't really know whether to change them or not and did not find an answer in the devguide (Might be worth documenting it ?) so I decided to change it :/.

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