add foobar() example to Function/Procedure section showing var parameters

Nim-for-C-programmers.md

@@ -240,6 +240,10 @@ int bar() {
 int baz(int x) {
   return x*2;
 }
+int foobar(int *x) {
+  return (++*x)*2;
+}
+
 </pre>
 </td>
 <td>
@@ -250,8 +254,12 @@ proc foo() =
 proc bar(): int =
   2
 
-proc baz(x: int) =
+proc baz(x: int): int =
   x*2
+
+proc foobar(x: var int): int =
+  inc(x)
+  result = x*2  # also valid
 </pre>
 </td>
 <td><b>Function/Procedure.</b>  </td>