Update 01-creating-app.md

[nguyentichduy123]

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[filipenevola]

Hey @nguyentichduy123, could you sign the CLA?

Thanks.

[denihs]

Hi @nguyentichduy123, thank you for your PR.

Just to give you context, it is supposed to be .app. as it is right now, but I get why you suggested the changes, the thing is, we already had this PR to fix those mistakes in the tutorial and I just merge it.

We are using .app. from now on, instead of .body. because in order to the new hot-module-replacement package work, all the templates must have a name, and the body is anonymous.

I will close this PR. If you have questions, let me know.