Update model-many.Rmd (missing word) (hadley#674)

model-many.Rmd

@@ -266,7 +266,7 @@ We see two main effects here: the tragedies of the HIV/AIDS epidemic and the Rwa
 1.  To create the last plot (showing the data for the countries with the
     worst model fits), we needed two steps: we created a data frame with
     one row per country and then semi-joined it to the original dataset.
-    It's possible avoid this join if we use `unnest()` instead of 
+    It's possible to avoid this join if we use `unnest()` instead of 
     `unnest(.drop = TRUE)`. How?
 
 ## List-columns