Update index.md

[DinoDichiara]

Fixe import flet_fastapi. Replace flet.fastappi to flet_fastapi

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[ndonkoHenri]

Thanks for your contribution! Can you consider the above comment by @CLAassistant ? :)

[FeodorFitsner]

But it's provided import flet.fastapi as flet_fastapi is added before, which was not in that function description. Correct function name though is flet.fastappi.app().

[ndonkoHenri]

Ah, that's right! There's is no alias in this case. Closing the PR...