Update examples/models/llama/README.md to fix build broken (#12393)

Fix the following error
```
CMake Error at tools/cmake/common/preset.cmake:122 (message):
  Use of 'EXECUTORCH_BUILD_EXTENSION_MODULE' requires
  'EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR'
Call Stack (most recent call first):
  tools/cmake/preset/default.cmake:300 (check_required_options_on)
  CMakeLists.txt:87 (include)
```

Signed-off-by: Jie Fu <[email protected]>

Author: DamonFool

examples/models/llama/README.md

@@ -277,6 +277,7 @@ cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
     -DCMAKE_INSTALL_PREFIX=cmake-out-android \
     -DCMAKE_BUILD_TYPE=Release \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
+    -DEXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \
     -DEXECUTORCH_ENABLE_LOGGING=1 \
@@ -381,6 +382,7 @@ cmake -DPYTHON_EXECUTABLE=python \
     -DEXECUTORCH_ENABLE_LOGGING=1 \
     -DCMAKE_BUILD_TYPE=Release \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
+    -DEXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \
     -DEXECUTORCH_BUILD_XNNPACK=OFF \