Fix brainfuck usage message

cpp/brainfuck/brainfuck.cpp

@@ -46,7 +46,7 @@ struct bf_context_t {
 
 int main(int argc, char* argv[]) {
     if (argc < 2) {
-        std::cout << "Usage: " << argv[0] << std::endl;
+        std::cout << "Usage: " << argv[0] << " [BRAINFUCK FILE]" << std::endl;
         return 1;
     }