Edited ch12_mining.adoc with Atlas code editor

ch12_mining.adoc

@@ -779,7 +779,7 @@ finding a hash that is less than the target becomes more and more
 difficult.
 
 ++++
-<p class="fix_tracking2">
+<p class="fix_tracking3">
 To give a simple analogy, imagine a game where players throw a pair of
 dice repeatedly, trying to throw less than a specified target. In the
 first round, the target is 12. Unless you throw double-6, you win. In