Fill out implicit steps for gcd example (#186)

To make the gcd example slightly easier to follow, add in implied steps.

Co-authored-by: Ben Fradet <benjamin.fradet@gmail.com>

Author: JacobArchambault

src/main/scala/scalatutorial/sections/TailRecursion.scala

@@ -49,7 +49,10 @@ object TailRecursion extends ScalaTutorialSection {
    *   gcd(21, 14)
    *   if (14 == 0) 21 else gcd(14, 21 % 14)
    *   if (false) 21 else gcd(14, 21 % 14)
+   *   gcd(14, 21 % 14)
    *   gcd(14, 7)
+   *   if (7 == 0) 14 else gcd(7, 14 % 7)
+   *   if (false) 14 else gcd (7, 14 % 7)
    *   gcd(7, 14 % 7)
    *   gcd(7, 0)
    *   if (0 == 0) 7 else gcd(0, 7 % 0)