foundations/12_calculator: Add clearer factorial explanation

[dantoms]

Because

The link to the full wikipedia page for factorials left learners looking at complex mathematical equations when simply giving a short explanation is far more beneficial

This PR

• Removed wikipedia link
• Added A simple explanation under a heading

Issue

Closes #XXXXX

Additional Information

https://discord.com/channels/505093832157691914/505093832157691916/1490444532624392326

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder

[mao-sz]

Thanks for this. I don't think we need a whole headed section on what a factorial is though. It looks a little odd visually, especially when it's just for 1 of the 6 tasks, and the section ends up including the last paragraph which is not related to it.

I think it'll be better to just replace the link with a resource that gives a simpler definition up front.