Fix koan

Fixes hadley#204

tidy.Rmd

@@ -354,7 +354,7 @@ There are two missing values in this dataset:
 * The return for the first quarter of 2016 is implicitly missing, because it
   simply does not appear in the dataset.
 
-One way to think about the difference is with this Zen-like koan: An implicit missing value is the presence of an absence; an explicit missing value is the absence of a presence.
+One way to think about the difference is with this Zen-like koan: An explicit missing value is the presence of an absence; an implicit missing value is the absence of a presence.
 
 The way that a dataset is represented can make implicit values explicit. For example, we can make the implicit missing value explicit putting years in the columns: