[CI][Spec Decode] Adjust threshold for flaky ngram spec decoding test again (#24771)

Signed-off-by: wwl2755 <wangwenlong2755@gmail.com>

tests/v1/e2e/test_spec_decode.py

@@ -117,9 +117,9 @@ def test_ngram_correctness(
                 print(f"ref_output: {ref_output.outputs[0].text}")
                 print(f"spec_output: {spec_output.outputs[0].text}")
 
-        # Heuristic: expect at least 68% of the prompts to match exactly
+        # Heuristic: expect at least 66% of the prompts to match exactly
         # Upon failure, inspect the outputs to check for inaccuracy.
-        assert matches >= int(0.68 * len(ref_outputs))
+        assert matches >= int(0.66 * len(ref_outputs))
         del spec_llm
         torch.cuda.empty_cache()
         cleanup_dist_env_and_memory()