fix zoom ratio for keyboard navigation

Dear Corentin,

this pull request should fix the zoom ratios.
In zoom 2.0.4 the following happened:

    plot(1:10)
    zm()
    # s H H s L L s
    # xlim=c(1, 10), ylim=c(1, 10)
    # xlim=c(-0.056, 11.056), ylim=c(1, 10)
    # xlim=c(0.909, 10.091), ylim=c(1, 10)

After applying this fix you will get:

    plot(1:10)
    zm()
    # s H H s L L s
    # xlim=c(1, 10), ylim=c(1, 10)
    # xlim=c(-0.056, 11.056), ylim=c(1, 10)
    # xlim=c(1, 10), ylim=c(1, 10)

Best wishes,

Sebastian