closes #112 - use correct casing in edit link

While I was doing some research I got curious as to what the `MarkdownWebPages` module was. It seems to have come originally from some code in the Nuget docs website as I can't find it anywhere else. Anyway they had a helper method to fix the casing of the whole url but we just needed the filename part so I extracted it and integrated it.

https://github.com/NuGet/NuGetDocs/blob/master/Docs.Core/MarkdownEngine/MarkdownWebPage.cs