Fix Markdown listing of function parameters

[domcorvasce]

Resolves #170

It's my first time diving into SwiftDoc codebase. Excuse me if I say something wrong.

The code which list parameters is atswift-doc/Supporting Types/Components/Documentation.swift:52.

List(of:  documentation.parameters) { parameter in
    Fragment { "\(parameter.name): \(parameter.description)" }
}

Parameter#description returns a string representation of the object, which includes the name and -.
I didn't look further into it, but obviously we were repeating the parameter name and adding an additional dash.

In the end, I replaced parameter.description with parameter.content (thanks to Documentation.swift:122).
It works now.

[mattt]

Great work, @domcorvasce! I fixed this for HTML with #156, but neglected to make the corresponding fix for CommonMark. Thanks for your help with this — I really appreciate it!