improve the background in 05.md

[mokajima]

Hi! I found the word render method is used in the Background section in the src/exercise/05.md and I've replaced that with function component. I think render method means the render method of a class component, is it correct? Nowadays, I think we mainly use function components so I've made those changes.

[kentcdodds]

@all-contributors please add @mokajima for docs

[allcontributors[bot]]

@kentcdodds

I've put up a pull request to add @mokajima! :tada:

[mokajima]

Thank you!

I still have a hard time talking about the function body like that instead of calling it a "render method"

I feel you 😅