BackOffDelay multiplies attempts from zero

[StounhandJ]

This MR addresses an issue/127 where the BackOffDelay function incorrectly multiplies the delay duration, not taking into account that the attempt count starts from 1, thus the first delay is initially multiplied twice. In the description for this function, what should increase with the sequence. But in fact, it's a passion from the very first call.

Steps to Reproduce:

package main

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/avast/retry-go/v4"
)

var count uint64 = 6
var currentTime = time.Now()

func action() error {
	fmt.Printf("Time: %v\n", time.Since(currentTime).Round(time.Second))
	currentTime = time.Now()
	if count == 0 {
		return nil
	}
	count -= 1
	return errors.New("something went wrong")
}

func main() {
	_ = retry.Do(action,
		retry.Attempts(3),
		retry.Delay(2*time.Second),
		retry.DelayType(retry.BackOffDelay),
		retry.LastErrorOnly(true),
		retry.Context(context.Background()))
}

Observed Output:

Time: 0s
Time: 4s
Time: 8s

Expected Output:

Time: 0s
Time: 2s
Time: 4s

[JaSei]

Thanks for this PR. I apologize for the delayed response.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (cbf4dbf) to head (6c62c20).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #130       +/-   ##
=========================================
- Coverage   90.86%      0   -90.87%     
=========================================
  Files           2      0        -2     
  Lines         230      0      -230     
=========================================
- Hits          209      0      -209     
+ Misses         17      0       -17     
+ Partials        4      0        -4     

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