I18n: Fix wording of accessibility text on revisions slider

[pedro-mendonca]

Update the wording of the accessibility text on the revisions slider about using the left and right arrow keys.
Fix to: Change revision by using the left and right arrow keys

Trac ticket: https://core.trac.wordpress.org/ticket/62306

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[peterwilsoncc]

Merged r59306 / 0fb4f61

Thank you!