Update hello-world exercise (#206)

Author: ErikSchierboom

exercises/hello-world/Example.cs

@@ -1,7 +1,4 @@
-public class HelloWorld
+public static class HelloWorld
 {
-    public static string Hello(string name)
-    {
-        return $"Hello, {name ?? "World"}!";
-    }
+    public static string Hello() => "Hello, World!";
 }

exercises/hello-world/HelloWorldTest.cs

@@ -4,22 +4,8 @@
 public class HelloWorldTest
 {
     [Test]
-    public void No_name()
+    public void Say_hi()
     {
-        Assert.That(HelloWorld.Hello(null), Is.EqualTo("Hello, World!"));
-    }
-
-    [Ignore("Remove to run test")]
-    [Test]
-    public void Sample_name()
-    {
-        Assert.That(HelloWorld.Hello("Alice"), Is.EqualTo("Hello, Alice!"));
-    }
-
-    [Ignore("Remove to run test")]
-    [Test]
-    public void Other_sample_name()
-    {
-        Assert.That(HelloWorld.Hello("Bob"), Is.EqualTo("Hello, Bob!"));
+        Assert.That(HelloWorld.Hello(), Is.EqualTo("Hello, World!"));
     }
 }