Fix the build broken of examples/llm_manual used in llm/getting-started.md (#12095)

Hi all,

While I was following
https://docs.pytorch.org/executorch/main/llm/getting-started , the cmake
build was broken due to the following error:
```
CMake Error at third-party/executorch/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):
  third-party/executorch/tools/cmake/preset/default.cmake:300 (check_required_options_on)
  third-party/executorch/CMakeLists.txt:87 (include)
```

It would be better to fix it.
Thanks.

Author: DamonFool

docs/source/llm/getting-started.md

@@ -362,6 +362,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 option(EXECUTORCH_ENABLE_LOGGING "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_MODULE "" ON)
+option(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_TENSOR "" ON)
 option(EXECUTORCH_BUILD_KERNELS_OPTIMIZED "" ON)
 
@@ -517,6 +518,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 option(EXECUTORCH_ENABLE_LOGGING "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_MODULE "" ON)
+option(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_TENSOR "" ON)
 option(EXECUTORCH_BUILD_KERNELS_OPTIMIZED "" ON)
 option(EXECUTORCH_BUILD_XNNPACK "" ON) # Build with Xnnpack backend

examples/llm_manual/CMakeLists.txt

@@ -13,6 +13,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 # Set options for executorch build.
 option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_MODULE "" ON)
+option(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR "" ON)
 option(EXECUTORCH_BUILD_EXTENSION_TENSOR "" ON)
 option(EXECUTORCH_BUILD_KERNELS_OPTIMIZED "" ON)
 option(EXECUTORCH_BUILD_XNNPACK "" ON) # Build with Xnnpack backend