See also: fluent/fluentd#3649

Before the fix and fluent/fluentd#3640 ,
the both of retry behavior and the document were wrong.

Actual behavior: `c + c + cb^1 + ... + cb^(k-1)` (k+1 retries totally)
Total calculation: `cb^(-1) + c + cb^1 + ... + cb^(k-2)`

Signed-off-by: Daijiro Fukuda <[email protected]>

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 <[email protected]>

Signed-off-by: Daijiro Fukuda <[email protected]>