[pytorch/executorch][diff_train] Fix tokenizer convert in xnnpack_README.md (#10003)

As titled.


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**GitHub Author**: Mengwei Liu <[email protected]> (Meta Employee)
**GitHub Repo**: [pytorch/executorch](https://github.com/pytorch/executorch)
**GitHub Pull Request**: [#10003](#10003)

Initially generated by: https://www.internalfb.com/intern/sandcastle/job/36028798801523578/

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diff-train-source-id: 20c7047

Differential Revision: [D72740940](https://our.internmc.facebook.com/intern/diff/D72740940/)

[ghstack-poisoned]

Author: kimishpatel

examples/demo-apps/android/LlamaDemo/docs/delegates/xnnpack_README.md

@@ -103,7 +103,7 @@ You may wonder what the ‘--metadata’ flag is doing. This flag helps export t
 
 * Convert tokenizer for Llama 2 and Llava (skip this for Llama 3.x)
 ```
-python -m extension.llm.tokenizer.tokenizer -t tokenizer.model -o tokenizer.bin
+python -m pytorch_tokenizers.tools.llama2c.convert -t tokenizer.model -o tokenizer.bin
 ```
 
 ### For LLaVA model