Update helloworld.c

[nixawk]

Thanks @tannuchoudhary. That's not a must code-style for main function. Please read nasa c style or apple code style, ...

• https://www.kernel.org/doc/html/latest/process/coding-style.html
• https://github.com/torvalds/linux/blob/master/scripts/Lindent
• https://www.gnu.org/software/indent/manual/