remove stale `domainPrefix` callout

**Describe the bug**
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from this page https://docs.amplify.aws/react/build-a-backend/auth/concepts/external-identity-providers/#configure-external-sign-in-backend

**To Reproduce**
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1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
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**Screenshots**
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**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
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