update 'ByRef' borrow types of closures + minor fixes

[JOE1994]

Hello! 🦀 This PR contains 2 commits.

1. update description of ByRef borrows of closures
   The current version of src/closure.md explains that ByRef borrows can either be shared, shallow, unique or mut (enum variants of rustc_middle::mir::BorrowKind).
   However, the actual enum type used for representing borrows from closures seems to be a different type with the same name. This PR updates the description for borrow types of closures. I found this out while tracking the type definitions.
   (InferBorrowKind -> UpvarCaptureMap -> UpvarCapture -> UpvarBorrow -> BorrowKind)

// rustc_middle::ty::BorrowKind
pub enum BorrowKind {
    ImmBorrow,
    UniqueImmBorrow,
    MutBorrow,
}

2. minor punctual/grammar fixes

Thank you for reviewing this PR 😃

P.S.
I'm really grateful to the authors who worked on the closure.md writeup! 😺
It was very helpful for understanding how borrows from closures are resolved.

[jyn514]

Thanks for working on this!

[jyn514]

Thank you again!