update docs (#3285)

Summary:
You no longer need export FLATC_EXECUTABLE

Pull Request resolved: #3285

Reviewed By: digantdesai

Differential Revision: D56507418

Pulled By: kirklandsign

fbshipit-source-id: 4ccab07c39c9fbfd4983c10f74fb8a345f24e030

Author: kirklandsign

examples/demo-apps/android/ExecuTorchDemo/README.md

@@ -39,7 +39,6 @@ We generate the model file for the ExecuTorch runtime in Android Demo App.
 For delegating DeepLab v3 to XNNPACK backend, please do the following to export the model:
 
 ```bash
-export FLATC_EXECUTABLE=$(realpath third-party/flatbuffers/cmake-android-out/flatc)
 python3 -m examples.xnnpack.aot_compiler --model_name="dl3" --delegate
 mkdir -p examples/demo-apps/android/ExecuTorchDemo/app/src/main/assets/
 cp dl3_xnnpack_fp32.pte examples/demo-apps/android/ExecuTorchDemo/app/src/main/assets/