add mkdir build in turtle readme before make

turtle/README.md

@@ -44,6 +44,7 @@ To use this library, you need to have the `raylib` library installed. Follow the
  To ensure `raylib` is compiled with position-independent code, follow these steps:
 
  ```bash
+ mkdir build
  cd ~/Desktop/raylib/build
  make clean
  cmake .. -DCMAKE_C_FLAGS="-fPIC" -DCMAKE_BUILD_TYPE=Release