fix(doc-typo): fix typo regarding hist directive

[TobbiAR]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngneat/effects/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

Renamed hostDirective to hostDirectives code example for effects-ng "Directive Effects".

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes

What is the current behavior?

Currently the docs example for directive effects contain a typo. The docs falsy states, that the effect should be added to hostDirective instead of hostDirectives.

Issue Number: #53

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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