HandOn: rename DisplayMultiply() DisplaySumOfMultiply()

@steff @vrouvrea

I've just begun to read the hands on (still in procedural...) but one of the modifications I have made is impactful for most hands on so I deemed it better to share it immediately.

There is a function which was called display_sum() which I renamed display_multiply()... which is not really better. I have renamed it DisplaySumOfMultiply(, which albeit ugly is correct. The notebook has been corrected as well (a display_sum slipped through and was unduly kept).

Others modifications are much more minor:

• I heeded a warning from CMake and changed the type of the variable which indicated which C++ standard was used.
• A typo was corrected.