ROCM Build Target or Build Instructions are incorrect

It seems that at some point the build target for ROCM was renamed from HIPBLAS to HIP. I'm not sure if this was inadvertent or deliberate, but either way, the build instructions for ROCM are incorrect. The README says to use:

```CMAKE_ARGS="-DGGML_HIPBLAS=on" pip install llama-cpp-python```

However, to successfully build ROCM, the command is actually:

```CMAKE_ARGS="-DGGML_HIP=on" pip install llama-cpp-python```

This is obviously a fairly trivial fix, but I haven't created a PR as I'm not sure if it's the docs or the build target name that is incorrect.