ring-buffer: Correct a grammatical error in a comment

The word "trace" begins with a consonant sound,
so "a" should be used instead of "an".

Link: https://lore.kernel.org/20241107095327.6390-1-liujing@cmss.chinamobile.com


Signed-off-by: liujing <liujing@cmss.chinamobile.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>