Fix basic cmake example

Fix #100 (closed) cf. https://cmake.org/cmake/help/latest/guide/tutorial/A%20Basic%20Starting%20Point.html

6-InRealEnvironment/2-FileStructure.ipynb

 ``` C++17
-// CMakeLists.txt
+# CMakeLists.txt
+# Ensure the cmake used is compatible with the CMake functions that are used
+# As CMAKE_CXX_STANDARD was introduced in CMake 3.1
+cmake_minimum_required(VERSION 3.1)
+
+# A project name is mandatory, preferably right after cmake_minimum_required call
+project(Hello)
+
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard; at least 17 is expected.")
 
 add_executable(hello