Minor syntax fix (#1720)

Meta.Rmd

@@ -22,7 +22,7 @@ You'll learn about metaprogramming and tidy evaluation in the following five cha
     metaprogramming story, briefly learning about all major components 
     and how they fit together to form a cohesive whole.
 
-1.  Chapter \@ref(expressions) shows that that all R code can be described as a 
+1.  Chapter \@ref(expressions) shows that all R code can be described as a 
     tree. You'll learn how to visualise these trees, how the rules of R's 
     grammar convert linear sequences of characters into these trees, and how to 
     use recursive functions to work with code trees.