llava : fix clip-model-is-vision flag in README.md (#5509)

* llava: fix clip-model-is-vision flag in README.md

This commit fixes the flag `--clip_model_is_vision` in README.md which
is does not match the actual flag:
```console
$ python convert-image-encoder-to-gguf.py --help
...
  --clip-model-is-vision
                        The clip model is a pure vision model
                        (ShareGPT4V vision extract for example)
```

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* llava: update link to vit config in README.md

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

---------

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>