docs(fundamentals): improve factory provider example code snippet

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

at our Discord a person asked about the OptionsProvider as they thought that this provider is built-in one

What is the new behavior?

a minor change that should help on clarifying that the class-based provider is just an example

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM