docs: SDL_Locale is a struct, not a function.

include/SDL3/SDL_locale.h

@@ -53,7 +53,7 @@ extern "C" {
  * would be "en"), and the country, if not NULL, will be an ISO-3166 country
  * code (so Canada would be "CA").
  *
- * \since This function is available since SDL 3.2.0.
+ * \since This struct is available since SDL 3.2.0.
  *
  * \sa SDL_GetPreferredLocales
  */