Update README.md - Precede "create-react-app" command with "npm"

[codewithvk]

[gitname]

Thanks for suggesting this! I am in the process of modernizing the instructions now and plan to replace that command with $ npx create-react-app ... along with removing create-react-app from the list of "prerequisites".

[gitname]

I forgot to close this PR when I overhauled the instructions to use $ npx create-react-app. Closing now.

@codewithvk, thanks again for suggesting this. When I overhauled the instructions, I also began adding contributor avatars to the bottom of the tutorial. I don't remember whether your suggestion played a role in my decision to use npx commands, but I'd rather err on the side of excessive credit.

If you're OK with me adding your avatar, you can let me know by saying so here, in which case I will add it.