Documentation: Samples need to be updated to .NET Core 2.0

[dustinsoftware]

I'll open a PR that moves away from project.json references

Here's the pages so far that I've found:

• https://mysql-net.github.io/MySqlConnector/tutorials/net-core-mvc/
• https://mysql-net.github.io/MySqlConnector/tutorials/best-practices/
• https://mysql-net.github.io/MySqlConnector/overview/installing/

[bgrainger]

Fixed by #373.