Docs: Correct the @return type of get_theme_mod().

This now matches the type of the `$value` parameter of `set_theme_mod()`.

Props mensmaximus.
Fixes #45319.



git-svn-id: https://develop.svn.wordpress.org/trunk@44613 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -890,7 +890,7 @@ function get_theme_mods() {
  *
  * @param string      $name    Theme modification name.
  * @param bool|string $default
- * @return string
+ * @return mixed
  */
 function get_theme_mod( $name, $default = false ) {
 	$mods = get_theme_mods();