Docs: Replace incorrect @private tags.

The `@private` tag does not exist and is not supported. To indicate that something is private when in the global namespace, `@access private` should be used instead.

Reference: [https://developer.wordpress.org/coding-standards/inline-documentation-standards/php/#phpdoc-tags PHP Documentation Standards: PHPDoc tags].

Props jrf.
See #59651.

git-svn-id: https://develop.svn.wordpress.org/trunk@57065 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme-previews.php

@@ -64,7 +64,7 @@ function wp_attach_theme_preview_middleware() {
  * @see https://github.com/WordPress/gutenberg/pull/41836.
  *
  * @since 6.3.0
- * @private
+ * @access private
  */
 function wp_block_theme_activate_nonce() {
 	$nonce_handle = 'switch-theme_' . wp_get_theme_preview_path();