Make fixes to helloworld tutorial
- Mention that we will build in the end, so user do not worry about it at the beginning of tutorial.
- Emphasize the lines appearing in the changed code.
- Fix highlighting of CMakeLists.txt file.
- Fix versions mentioned (5 -> 6).
- Remove mentioning of helloworld_SRCS variable, as it is not there.
- Separate the Build section to several subsections.
- Reformat the source file: removed unneeded whitespace and unnecessary difference.