ring-buffer: Fix mispelling of Calculate

It's not "Caculate".

Link: http://lkml.kernel.org/r/20181101154640.23162-1-tiny.windzz@gmail.com



Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>