Update csharp-7-3.md (#6128)

Author: BillWagner

docs/csharp/whats-new/csharp-7-3.md

@@ -66,7 +66,7 @@ class C
 {
     static S s = new S();
 
-    public void M()
+    unsafe public void M()
     {
         fixed (int* ptr = s.myFixedField)
         {