Fix typo (#257)

tidy.Rmd

@@ -41,7 +41,7 @@ There are three interrelated rules which make a dataset tidy:
 
 1.  Each variable must have its own column.
 1.  Each observation must have its own row.
-1.  Each value much have its own cell.
+1.  Each value must have its own cell.
 
 Figure \@ref(fig:tidy-structure) shows the rules visually.