tutorial: clearer explanation of freezing.

- "Lending an immutable pointer" might be confusing. It was not discussed why borrowed pointers are immutable in the first place.
- Make it clear that the borrowed pointers are immutable even if the variable was declared with `mut`.
- Make it clear that we cannot even assign anything to the variable while its value is being borrowed.

tutorial: change "--" to an em-dash.

tutorial: change instances of "--" to em-dash.