Minor typos in the documentation for git checkout <branch>

["Rory O’Kane" <rory@roryokane.com>]

I noticed some errors in one paragraph of the documentation for the <branch>
argument of git-checkout. A convenient link to that documentation:
https://git-scm.com/docs/git-checkout#git-checkout-ltbranchgt

This is the current AsciiDoc source for that paragraph:

    You can use the `"@{-N}"` syntax to refer to the N-th last
    branch/commit checked out using "git checkout" operation. You may
    also specify `-` which is synonymous to `"@{-1}`.

That source comes from Documentation/git-checkout.txt:
https://github.com/git/git/blob/77556354bb7ac50450e3b28999e3576969869068/Documentation/git-checkout.txt#L290-L292

I suggest changing that source to this:

    You can use the `"@{-<n>}"` syntax to refer to the <n>th last
    branch/commit checked out using the "git checkout" operation. You
    may also specify `"-"`, which is synonymous to `"@{-1}"`.

These are the changes:

- Added the missing closing double quote to `"@{-1}` at the end.
- Added the word “the” before ‘"git checkout” operation’.
- Added quotes around “-”, to make it more visible and consistent with the
  other syntax in the paragraph.
- Added comma before “which”.
- For consistency, used the angle bracket style of placeholder also used by
  the explanation of the “@{-<n>}, e.g. @{-1}” syntax (see link) within the
  documentation for gitrevisions (see link).
  [gitrevisions source]:
https://github.com/git/git/blob/77556354bb7ac50450e3b28999e3576969869068/Documentation/revisions.txt#L94-L96
  [gitrevisions rendered]:
https://git-scm.com/docs/gitrevisions#gitrevisions-em-ltngtemegem-1em

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