Fix wrong description of total retry time

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 <fukuda@clear-code.com>

buffer/README.md

@@ -41,7 +41,7 @@ A chunk can fail to be written out to the destination for a number of reasons. T
 By default, Fluentd increases the wait interval exponentially for each retry attempt. For example, assuming that the initial wait interval is set to 1 second and the exponential factor is 2, each attempt occurs at the following time points:
 
 ```text
-1 2   4       8               16
+0 1   3       7               15
 x-x---x-------x---------------x-------------------------
 │ │   │       │               └─  4th retry (wait = 8s)
 │ │   │       └─────────────────  3th retry (wait = 4s)

configuration/buffer-section.md

@@ -546,7 +546,8 @@ With `exponential_backoff`, `retry_wait` interval will be calculated as below:
 * c: constant factor, `@retry_wait`
 * b: base factor, `@retry_exponential_backoff_base`
 * k: number of retry times
-* total retry time: `c + c * b^1 + (...) + c*b^k = c*b^(k+1) - 1`
+* total retry time: `c + c*b^1 + (...) + c*b^(k-1) = c*(b^k - 1) / (b - 1)`
+  * = `2^k - 1` by default
 
 If this article is incorrect or outdated, or omits critical information, please [let us know](https://github.com/fluent/fluentd-docs-gitbook/issues?state=open). [Fluentd](http://www.fluentd.org/) is an open-source project under [Cloud Native Computing Foundation \(CNCF\)](https://cncf.io/). All components are available under the Apache 2 License.