fix(admin-settings): correct variable typo

includes/admin/class-algolia-admin-page-settings.php

@@ -170,7 +170,7 @@ public function index_name_prefix_callback() {
 		$index_name_prefix = $settings->get_index_name_prefix();
 		$disabled_html = $settings->is_index_name_prefix_in_config() ? ' disabled' : '';
 ?>
-		<input type="text" name="algolia_index_name_prefix" value="<?php echo esc_attr( $setting ); ?>" <?php echo esc_html( $disabled_html ); ?>/>
+		<input type="text" name="algolia_index_name_prefix" value="<?php echo esc_attr( $index_name_prefix ); ?>" <?php echo esc_html( $disabled_html ); ?>/>
 		<p class="description" id="home-description"><?php esc_html_e( 'This prefix will be prepended to your index names.', 'algolia' ); ?></p>
 <?php
 	}