Fix count to actually _be_ a count

Also, start from 1 in the positive direction, instead of 5, since that
is more how the thing works (sorry, mathematician brain).

Additionally, adds some extra information in the comments:
information about how long the code would take if one actually sets
count to 2,5 * 10^12, and adding a plus in front of the 4/1 to show that
that is in fact the first element of the "plus" side.

Finally, add an (floating point) error to the debug print.

rivendell/elrond.zig

@@ -1150,7 +1150,7 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "108_threading2.zig",
-        .output = "PI ≈ 3.14159265",
+        .output = "PI ≈ 3.14159265 (error = 1.0e-9)",
     },
     .{
         .main_file = "109_files.zig",