🧹 [Maintenance]: Fix static middleware config typo in docs

[grivera64]

Maintenance Task Description

There is a typo of the data type of the field CacheDuration. On docs.gofiber.io, the CacheDuration type is string, when the actual code says time.Duration.

Impact on the Project

Fixing this typo will cause less confusion when reading the docs.

Additional Context (optional)

The markdown file to be modified is docs/middleware/static.md.

I can work on this simple fix.

Checklist:

• I have confirmed that this maintenance task is currently not being addressed.
• I understand that this task will be evaluated by the maintainers and prioritized accordingly.
• I am available to provide further information if needed.