Edit install-and-use-a-package-using-the-dotnet-cli.md -- remove source of warning (#3220)

The sample code
```csharp
public class Account
{
    public string Name { get; set; }
    public string Email { get; set; }
    ...
}
```
leads to warnings:
```
$ dotnet run
Nuget.Quickstart/Program.cs(7,23): warning CS8618: Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [Nuget.Quickstart/Nuget.Quickstart.csproj]
Nuget.Quickstart/Program.cs(8,23): warning CS8618: Non-nullable property 'Email' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [Nuget.Quickstart/Nuget.Quickstart.csproj]
```

This commit applies a small edit so that `dotnet run` does not warn.

The warnings are not a big deal. They are a small distraction. I propose this edit in case the small distraction is worth removing.

Author: zipperer

docs/quickstart/install-and-use-a-package-using-the-dotnet-cli.md

@@ -67,8 +67,8 @@ You can install NuGet packages into a .NET project. For this walkthrough, create
     {
         public class Account
         {
-            public string Name { get; set; }
-            public string Email { get; set; }
+            public string? Name { get; set; }
+            public string? Email { get; set; }
             public DateTime DOB { get; set; }
         }
         internal class Program