Fix wrong number of retry times

See also: fluent/fluentd#3649

Before the fix, the number was mistakenly 19.
This fixes it to 18.
(The document was also wrong.)

n-th retry is triggered at `2^n - 1` seconds elapsed since the first flush.
So 18-th retry is at 262143 seconds elapsed and this exceeds 72 hours (259200 seconds).
If the next retry is going to exceed this time limit, the last retry will be
made at exactly this time limit.
So the last time is 18.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

output/README.md

@@ -204,7 +204,7 @@ Default: `72` \(hours\)
 
 #### `retry_max_times`
 
-The maximum number of times to retry to flush while failing. If `retry_timeout` is the default, the number is 17 with exponential backoff.
+The maximum number of times to retry to flush while failing. If `retry_timeout` is the default, the number is 18 with exponential backoff.
 
 Default: `nil`