build: bump cmake_minimum_required in example

[KowalskiThomas]

What is this PR?

This bumps the minimum CMake version for example because it is making AppVeyor unhappy.

Builds are failing on my other PRs (example) with this error message.

Build started
git clone -q https://github.com/Tencent/rapidjson.git C:\projects\rapidjson-0fdqj
git fetch -q origin +refs/pull/2378/merge:
git checkout -qf FETCH_HEAD
git submodule update --init --recursive
Submodule 'thirdparty/gtest' (https://github.com/google/googletest.git) registered for path 'thirdparty/gtest'
Cloning into 'C:/projects/rapidjson-0fdqj/thirdparty/gtest'...
Submodule path 'thirdparty/gtest': checked out 'ba96d0b1161f540656efdaed035b3c062b60e006'
cmake -H. -BBuild/VS -G "Visual Studio %VS_VERSION%" -DCMAKE_GENERATOR_PLATFORM=%VS_PLATFORM% -DCMAKE_VERBOSE_MAKEFILE=ON -DBUILD_SHARED_LIBS=true -DRAPIDJSON_BUILD_CXX11=%CXX11% -DRAPIDJSON_BUILD_CXX17=%CXX17% -DRAPIDJSON_BUILD_CXX20=%CXX20% -DRAPIDJSON_USE_MEMBERSMAP=%MEMBERSMAP% -Wno-dev
-- The CXX compiler identification is MSVC 19.29.30159.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.29.30133/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Doxygen: C:/Tools/Doxygen/doxygen.exe (found version "1.14.0") found components: doxygen dot
CMake Error at example/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.
  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
-- Configuring incomplete, errors occurred!