fix link in stacking regressor

[codecov[bot]]

Codecov Report

Base: 77.45% // Head: 77.46% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (76f5174) compared to base (75c5284). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #996 +/- ## ========================================== + Coverage 77.45% 77.46% +0.01% ========================================== Files 198 198 Lines 11171 11171 Branches 1406 1406 ========================================== + Hits 8652 8654 +2 + Misses 2305 2304 -1 + Partials 214 213 -1 ``` | [Impacted Files](https://codecov.io/gh/rasbt/mlxtend/pull/996?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/regressor/stacking\_cv\_regression.py](https://codecov.io/gh/rasbt/mlxtend/pull/996/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9yZWdyZXNzb3Ivc3RhY2tpbmdfY3ZfcmVncmVzc2lvbi5weQ==) | `94.44% <ø> (ø)` | | | [mlxtend/evaluate/counterfactual.py](https://codecov.io/gh/rasbt/mlxtend/pull/996/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9jb3VudGVyZmFjdHVhbC5weQ==) | `100.00% <0.00%> (+6.89%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.