docs: clarify now() helper usage in signed URL example

[Amirhf1]

Description
Improved clarity of the temporarySignedRoute example by adding an inline
comment explaining that now() is a Laravel helper function.

Motivation
New Laravel developers might be confused seeing now() without any import
statement. This small addition helps them understand that it's a built-in
Laravel helper, not requiring explicit Carbon import.

Changes

• Added inline comment to clarify now() helper function usage
• Helps beginners understand Laravel's helper functions better

[shaedrich]

Despite the description saying, the PR explains this, I cannot see the actual changes explicitly reflecting that. It just results in an unused import.

Furthermore, helper functions are a core concept of the framework, not something specifically tied to signed URLs. Feeling the need to add this here, would require us to add this everywhere where helpers are used in the documentation—which sounds like quite an overhead as compared to just consulting the /helpers docs page.