Update NDK version in using-executorch-building-from-source.md (pytorch#8812)

Use NDK 27 because some early version doesn't support BF16.

Author: kirklandsign

docs/source/using-executorch-building-from-source.md

@@ -218,7 +218,7 @@ Assuming Android NDK is available, run:
 mkdir cmake-android-out && cd cmake-android-out
 
 # point -DCMAKE_TOOLCHAIN_FILE to the location where ndk is installed
-cmake -DCMAKE_TOOLCHAIN_FILE=/Users/{user_name}/Library/Android/sdk/ndk/25.2.9519653/build/cmake/android.toolchain.cmake  -DANDROID_ABI=arm64-v8a ..
+cmake -DCMAKE_TOOLCHAIN_FILE=/Users/{user_name}/Library/Android/sdk/ndk/27.2.12479018/build/cmake/android.toolchain.cmake  -DANDROID_ABI=arm64-v8a ..
 
 cd  ..
 cmake --build  cmake-android-out  -j9