ring-buffer: Fix mispelling of Calculate

It's not "Caculate".

Link: http://lkml.kernel.org/r/20181101154640.23162-1-tiny.windzz@gmail.comSigned-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer_benchmark.c

@@ -362,7 +362,7 @@ static void ring_buffer_producer(void)
 			hit--; /* make it non zero */
 		}
 
-		/* Caculate the average time in nanosecs */
+		/* Calculate the average time in nanosecs */
 		avg = NSEC_PER_MSEC / (hit + missed);
 		trace_printk("%ld ns per entry\n", avg);
 	}