doc/APIchanges: add missing marker for release 4.3

Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -51,6 +51,8 @@ API changes, most recent first:
   Add av_opt_child_class_iterate() and AVClass.child_class_iterate().
   Deprecate av_opt_child_class_next() and AVClass.child_class_next().
 
+-------- 8< --------- FFmpeg 4.3 was cut here -------- 8< ---------
+
 2020-06-05 - ec39c2276a - lavu 56.50.100 - buffer.h
   Passing NULL as alloc argument to av_buffer_pool_init2() is now allowed.