Fix title of custom ambassador notebook, fix prerequistes typo

[MarcoGorelli]

I was slightly too late putting in a review for #896 - the 'prerequistes' typos appears throughout the notebooks, so I've fixed it for the rest of them.

Also, the Ambassador Canary notebook was being linked to twice, and the title for examples/ambassador_custom was the same as that for examples/ambassador_header.

[seldondev]

Hi @MarcoGorelli. Thanks for your PR.

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[ukclivecox]

/ok-to-test

[ukclivecox]

Thanks for this!

[ukclivecox]

/approve

[seldondev]

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