Course.model.schema has old values for _accessibilityToggleTextOn and _accessibilityToggleTextOff

[dancgray]

_accessibilityToggleTextOn should be accessibilityToggleTextOn

_accessibilityToggleTextOff should be accessibilityToggleTextOff

This will affect the translation of an export from the authoring tool for these values.

[moloko]

As it happens, the code is set up to check for the underscored versions as well so that we didn't break backwards compatibility when we changed this - but we should of course get this corrected for consistency/correctness