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This cl changes one string in Keyboard Shortcut Helper to make it
more clear to the users.

Bug: b/74186947
Test: manually tested.
Change-Id: I4af4d7a99c9a279a775309e58584c06e63f7a136
Reviewed-on: https://chromium-review.googlesource.com/964586Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543454}