Fix typo in GoDoc comment for NewConstant function (#31)

The NewConstant function will panic if the time duration is less than or
equal to zero.

The GoDoc comment for this function needs to be updated to say that it
will panic if the given base is "less than or equal to zero" instead of
"less than zero".

Author: domelzer

backoff_constant.go

@@ -13,7 +13,7 @@ func Constant(ctx context.Context, t time.Duration, f RetryFunc) error {
 
 // NewConstant creates a new constant backoff using the value t. The wait time
 // is the provided constant value. It panics if the given base is less than
-// zero.
+// or equal to zero.
 func NewConstant(t time.Duration) Backoff {
 	if t <= 0 {
 		panic("t must be greater than 0")