Update value.h.md

doc/api/value.h.md

@@ -30,6 +30,7 @@ main(void)
   mrb_funcall(mrb, obj, "method_name", 1, mrb_float_value(mrb, f));
   fclose(fp);
   mrb_close(mrb);
+  return 0;
 }
 
 ```
@@ -77,6 +78,7 @@ main(void)
   mrb_funcall(mrb, obj, "method_name", 1, mrb_fixnum_value(i));
   fclose(fp);
   mrb_close(mrb);
+  return 0;
 }
 
 ```
@@ -124,6 +126,7 @@ main(void)
   mrb_funcall(mrb, obj, "method_name", 1, mrb_nil_value());
   fclose(fp);
   mrb_close(mrb);
+  return 0;
 }
 
 ```
@@ -170,6 +173,7 @@ main(void)
   mrb_funcall(mrb, obj, "method_name", 1, mrb_false_value());
   fclose(fp);
   mrb_close(mrb);
+  return 0;
 }
 
 ```
@@ -217,6 +221,7 @@ main(void)
   mrb_funcall(mrb, obj, "method_name", 1, mrb_true_value());
   fclose(fp);
   mrb_close(mrb);
+  return 0;
 }
 
 ```