Minor touchup to height conversion prog

d/prog-lang-book/src/height_conversions.d

@@ -12,7 +12,7 @@ void main() {
     // Loop'n write
     foreach (feet; 5 .. 7) {
         foreach (inches; 0 .. inchesPerFoot) {
-            writeln(feet, "'", inches, "''\t",
+            writefln("%s'%s''\t%s", feet, inches,
                 (feet * inchesPerFoot + inches) * cmPerInch);
         }
     }