Some typos, init with braces and explanation for semicolon at the end of struct

• Most of the modifications are typos.
• In classes, I use braces to initialize a default attribute value right after it is explained to do so (instead of =)
• I changed 2 links that were mentioning parshift, but the links were to isocpp.
• I added a paragraph to explain why we put a semicolon at the end of a struct and class (maybe because I see this error too often with ENSTA students)
• I added a note about prefixing an attribute with an underscore that is reserved to the standard
• Some code cells were in markdown