ring-buffer: Fix typo in comment

Fix typo of the word 'been'

Link: http://lkml.kernel.org/r/20180518203130.2011-1-gomonovych@gmail.com

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>