[Learn-C] Outdated information about bools not being in C

[ghost]

Bools have been implemented in C with the stdbool.h header since C99, one should use that instead of defining a separate boolean variable by themselves

[dyskos]

Why hasn't this been updated yet? It's been nearly a year since its mention. That's nearly a year that beginners may have been misled by information that's quite literally 23 years out of date.

Also, it should be noted that -- per the C standard -- functions that take no arguments should have a void in its parameter list.

[ronreiter]

@dyskos would you like to send a PR?