Rename generic type parameter in Subtyping chapter

[qsantos]

Like several issues (#124, #262, #305, #339) have pointed before, the chapter “Subtyping and Variance” can easily become confusing. I have experienced that as well, and I have noticed that this was partially caused by T used both in &'a T and as the &'a T itself in &mut &'a T (thus written &mut T).

To make things clearer, this PR renames the generic type parameter of the function R, since it is intended to be a reference in the examples. &'a T stays unchanged, but R is used when &'a T itself is intended; for instance, the more generic variant of &mut &'a T is thus &mut R.

I have also slightly changed some explanations to clarify them and take into account this new naming.