Add prompt to clarify parameter as template list instead of file

[f1ndr]

This commit adds a prompt to inform users when their input is interpreted as a list of templates, rather than a single template file. This addresses potential confusion where a user may unknowingly pass a parameter intended to be a single template file but is treated as a list. The added message specifies that the file extension should be either yaml or json to be recognized as a single template file.

Proposed changes

Problem
Today is my first day using Nuclei. I ran into an issue when using a file with a .yml extension. The application threw an error (see screenshot). After spending considerable time debugging, I found out that Nuclei was treating my file as a list of templates rather than a single template file.

Solution
To address this, I've modified the code to add an info message. The new message informs the user that the application interprets their input as a list of templates. It also specifies that in order to be recognized as a single template file, the file extension should be either .yaml or .json (see screenshot).

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)