Fix TextInput maxLength documentation

[huytdps13400]

Summary

• correct the documented TextInput maxLength prop type from string to number
• align the generated documentation with the existing public Flow type and HTML contract

Fixes #2855.

Verification

• npm run build -w react-native-web-docs: passed; generated TextInput page renders maxLength as ?number
• npm run test: Flow, Prettier, ESLint, 46 DOM suites / 712 tests passed, 3 Node suites / 6 tests passed
• git diff --check: passed

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