Make the runnable example actually work (#4202)

The runnable example added yesterday unfortunately did not work. Changed the code so that it actually works. Added also another features of an interpolated string. 
I noticed during Tsoding's stream and also from personal experience, that discovering features often happen by looking for something else.

Author: schlupa

articles/ctod.dd

@@ -646,9 +646,13 @@ ulong[string] map = ["hello"    : 1616515,
                      "goodbye"  : 42,
                      "maybe"    : ulong.max
                     ];
-string s = "goodbye";
-
-writeln("number is ", map[s]);
+unittest
+{
+    import std.stdio;
+    writeln("number is ", map["goodbye"]);
+    writeln(i"map contains $(str.length) elements");
+    writeln("map is ", map);
+}
 ----------------------------
 )