color: Fix fallout from the GtkTemplate port

Correct a typoed variable name so that calibration works again, instead
of crashing.

GLib-GObject-WARNING **: 15:52:50.561: invalid cast from 'GtkBox' to 'GtkEntry'
11 jobs for wip/kalev/color-entry_calib_title in 3 minutes and 43 seconds (queued for 3 seconds)
Status Job ID Name Coverage
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  Test
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passed #104647
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  Review
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