Readme: Fix link anchors for ScriptableObjectInstaller

[krisrok]

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• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [ ] No compiler errors or warnings

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What is the current behavior?

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What is the new behavior?

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Does this introduce a breaking change?

• [ ] Yes
• [ ] No

Other information

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On which Unity version has this been tested?

• [ ] 2020.4 LTS
• [ ] 2020.3
• [ ] 2020.2
• [ ] 2020.1
• [ ] 2019.4 LTS
• [ ] 2019.3
• [ ] 2019.2
• [ ] 2019.1
• [ ] 2018.4 LTS

Scripting backend:

• [ ] Mono
• [ ] IL2CPP

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