move change from V3.8 to V3.9 (#3869)

* move change from V3.8 to V3.9

#3852 adds the new feature to the V3.8 changelog while it should go in the V3.9 changelog.

* fix small typo

* move the change a little down in the changelog

RELEASE-NOTES.md

@@ -8,6 +8,7 @@
 - `DEMetropolisZ`, an improved variant of `DEMetropolis` brings better parallelization and higher efficiency with fewer chains with a slower initial convergence. This implementation is experimental. See [#3784](https://github.com/pymc-devs/pymc3/pull/3784) for more info.
 - Notebooks that give insight into `DEMetropolis`, `DEMetropolisZ` and the `DifferentialEquation` interface are now located in the [Tutorials/Deep Dive](https://docs.pymc.io/nb_tutorials/index.html) section.
 - Add `fast_sample_posterior_predictive`, a vectorized alternative to `sample_posterior_predictive`.  This alternative is substantially faster for large models.
+- GP covariance functions can now be exponentiated by a scalar. See PR [#3852](https://github.com/pymc-devs/pymc3/pull/3852)
 - `sample_posterior_predictive` can now feed on `xarray.Dataset` - e.g. from `InferenceData.posterior`. (see [#3846](https://github.com/pymc-devs/pymc3/pull/3846))
 - `SamplerReport` (`MultiTrace.report`) now has properties `n_tune`, `n_draws`, `t_sampling` for increased convenience (see [#3827](https://github.com/pymc-devs/pymc3/pull/3827))
 - `pm.sample` now has support for adapting dense mass matrix using `QuadPotentialFullAdapt` (see [#3596](https://github.com/pymc-devs/pymc3/pull/3596), [#3705](https://github.com/pymc-devs/pymc3/pull/3705) and [#3858](https://github.com/pymc-devs/pymc3/pull/3858))
@@ -30,7 +31,6 @@
 - Distinguish between `Data` and `Deterministic` variables when graphing models with graphviz. PR [#3491](https://github.com/pymc-devs/pymc3/pull/3491).
 - Sequential Monte Carlo - Approximate Bayesian Computation step method is now available. The implementation is in an experimental stage and will be further improved.
 - Added `Matern12` covariance function for Gaussian processes. This is the Matern kernel with nu=1/2.
-- Added exponentiation of a covariance function with a scalar. See PR[#3852](https://github.com/pymc-devs/pymc3/pull/3852)
 - Progressbar reports number of divergences in real time, when available [#3547](https://github.com/pymc-devs/pymc3/pull/3547).
 - Sampling from variational approximation now allows for alternative trace backends [#3550].
 - Infix `@` operator now works with random variables and deterministics [#3619](https://github.com/pymc-devs/pymc3/pull/3619).