refactor: Use USAGE_CODE constant

I missed a case in #2604.  This will help people discover the
documentation describing why we chose the code we did.

src/build/app/mod.rs

@@ -2064,7 +2064,7 @@ impl<'help> App<'help> {
 
                 drop(self);
                 drop(e);
-                safe_exit(2);
+                safe_exit(USAGE_CODE);
             }
 
             drop(self);