Update dotnet-core-2.1.md

[federicorinaldi]

Visual Studio 2017 15.7 has been released now so this document is outdated

[AngelosP]

Thank you for the heads up. I'm not 100% sure why we think it's outdated, can you please give me a hint? Don't forget that even though VS 15.7 is out, that doesn't mean 100% of our customers have moved over to it. If a customer is still using VS15.5 or VS15.6 we want them to be able to read this .md file and understand that they should upgrade to VS15.7 or later.

[federicorinaldi]

Exactly! If you look at the PR, you can see that I only updated the section for VS 15.7 the rest (for VS 15.6 and 15.5) are still there. The change I've made is basically to allow the people to upgrade to VS 15.7 or later without pointing them to the Preview page for download

[AngelosP]

I'm really sorry, I almost never use GitHub PRs and I just didn't read it right :(