Fix link for tools/perplexity in README.md

Author: am17an

README.md

@@ -459,7 +459,7 @@ To learn more about model quantization, [read this documentation](tools/quantize
 
     </details>
 
-[^1]: [tools/perplexity/README.md](./tools/perplexity/README.md)
+[^1]: [https://github.com/ggml-org/llama.cpp/blob/master/tools/perplexity/README.md](./tools/perplexity/README.md)
 [^2]: [https://huggingface.co/docs/transformers/perplexity](https://huggingface.co/docs/transformers/perplexity)
 
 ## [`llama-bench`](tools/llama-bench)