hotfix: Damped ETS

[MMenchero]

Description

This fixes the damped-trend forecast recursion in ETS models.

For a damped additive trend, the correct trend coefficient at horizon $h$ is

$$ \phi + \phi^2 + \cdots + \phi^h. $$

Since i starts at zero, changing pow(phi, i + 1) to pow(phi, i + 2) after horizon 1 produces the correct horizon-2 coefficient, $\phi + \phi^2$, instead of the incorrect $2\phi$.

The shared recursion also affects damped multiplicative trends.

The fable equivalent script uses pow(phi, i+2). See https://github.com/tidyverts/fable/blob/3fe8346a5506ba3f262f365de9b0d293e217f2bc/src/etscalc.c#L236

[codspeed-hq]

Merging this PR will not alter performance

✅ 38 untouched benchmarks

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_{Comparing hotfix/damped-ets (2fe73d7) with main (a580cf9)}