Fix error in extend doc (sfBuildersProvider -> sfBuilderProvider)

[chargrove]

Convert non-existing sfBuildersProvider to sfBuilderProvider

Description

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Fixes Related issues

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Checklist

• [x] I have read and understand the CONTRIBUTIONS.md file
• [x] I have searched for and linked related issues
• [x] I have created test cases to ensure quick resolution of the PR is easier
• [x] I am NOT targeting main branch
• [x] I did NOT include the dist folder in my PR

@json-schema-form/angular-schema-form-lead

[Anthropic]

@chargrove thank you, appreciate your help :)