Update AdditionOperator.cs (#975)

Author: pkulikov

csharp/language-reference/operators/AdditionOperator.cs

@@ -24,10 +24,10 @@ private static void NumericAddition()
         private static void StringConcatenation()
         {
             // <SnippetAddStrings>
-            Console.WriteLine("Forgot " + " white space");
+            Console.WriteLine("Forgot" + "white space");
             Console.WriteLine("Probably the oldest constant: " + Math.PI);
             // Output:
-            // Forgot white space
+            // Forgotwhite space
             // Probably the oldest constant: 3.14159265358979
             // </SnippetAddStrings>