Make C code more recognizably C (#331)

src/ffi.md

@@ -281,7 +281,7 @@ We'll create a C file to call the `hello_from_rust` function and compile it by `
 C file should look like:
 
 ```c
-int main() {
+int main(void) {
     hello_from_rust();
     return 0;
 }