RFR: git-pr-checkout: better helptext

[Erik Helin]

Hi all,

please review this patch that slightly updates the helptext for the `-b, --branch` flag for `git pr checkout` to make
it align more with Git's helptext for the `-b, --branch` option to `git checkout`.

Thanks,
Erik

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Commit messages:
 - git-pr-checkout: better helptext

Changes: https://git.openjdk.java.net/skara/pull/792/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=792&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/792.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/792/head:pull/792

PR: https://git.openjdk.java.net/skara/pull/792