wrong git command in makefile

Instruction from makefile should be 

```
git clone --recurse-submodules https://github.com/XUANTIE-RV/csi-nn2.git"
```

instead of 

```
git clone https://github.com/XUANTIE-RV/csi-nn2.git
```

Because the "dlpack" sub-module won't be cloned with the current command and will generate error on make for missing dependencies.