Clarify int main(void) example

The semicolon can elicit a syntax error from C compilers such as MSVC.
Specifying a return is useful: in ISO C ("C99") and later, the `return 0;` is implied.
However, in ANSI C (AKA "C89") the compiler is not required to add `return 0;`.
This document references ANSI C, so clarity on the matter and consistent style is best.
Also, it is best to identify the struct with the bit layout in question.

Author: workingjubilee

docs/c-language/c-bit-fields.md

@@ -56,15 +56,16 @@ struct mybitfields
     unsigned short c : 7;
 } test;
 
-int main( void );
+int main( void )
 {
     test.a = 2;
     test.b = 31;
     test.c = 0;
+    return 0;
 }
 ```
 
-the bits would be arranged as follows:
+the bits of `test` would be arranged as follows:
 
 ```
 00000001 11110010