Use the correct version for the \since field

include/SDL3/SDL_stdinc.h

@@ -2556,7 +2556,7 @@ extern SDL_DECLSPEC Uint32 SDLCALL SDL_StepUTF8(const char **pstr, size_t *pslen
  *
  * \threadsafety It is safe to call this function from any thread.
  *
- * \since This function is available since SDL 3.0.0.
+ * \since This function is available since SDL 3.1.5.
  */
 extern SDL_DECLSPEC Uint32 SDLCALL SDL_StepBackUTF8(const char *start, const char **pstr);