Tests: Update style of c++ code snippets in Tutorial directions

Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

00:24:28

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:55:10

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:02:03

passed cmake-moonshade-osx-debug-ninja

00:10:46

passed cmake-moonshade-osx-release-makefiles

00:12:55

passed cmake-terfin-windows-release-vs2017

00:45:56

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager