Update Themer migration guide in README (#9244)
Update Themer migration guide in README (#9244)

Updates the Themer-to-Theming-Extension migration guide in the themers README to reference a themer that has not yet been deleted. 
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios