C#: Minor improvements to interest-is-interesting exercise's concept introduction

Exercism Support
1

I made a few small tweaks to the interest-is-interesting C# exercise’s introduction and opened a PR:

The bot auto-closed it and suggested posting it in the forum for discussion, so here it is.

2

Changes look good. I’ve reopened. Thanks.

3

This could be moved to C# - Exercism as well, so the csharp community will note it. They may not monitor this category.