Remove "or // to use ContentAI"

function custom_block_editor_script() {
    ?>
    <script type="text/javascript">
        wp.domReady( function() {
            wp.hooks.addFilter(
                'blocks.registerBlockType',
                'mytheme/custom-placeholder',
                function( settings, name ) {
                    if ( name === 'core/paragraph' && settings.attributes.placeholder ) {
                        settings.attributes.placeholder.default = 'Type / to choose a block';
                    }
                    return settings;
                }
            );
        } );
    </script>
    <?php
}
add_action( 'admin_footer', 'custom_block_editor_script' );