Non-syntactic reminder. Fixes hadley#509

tidy.Rmd

@@ -148,7 +148,7 @@ table4a %>%
   gather(`1999`, `2000`, key = "year", value = "cases")
 ```
 
-The columns to gather are specified with `dplyr::select()` style notation. Here there are only two columns, so we list them individually. Note that "1999" and "2000" are non-syntactic names so we have to surround them in backticks. To refresh your memory of the other ways to select columns, see [select](#select).
+The columns to gather are specified with `dplyr::select()` style notation. Here there are only two columns, so we list them individually. Note that "1999" and "2000" are non-syntactic names (because they don't start with a letter) so we have to surround them in backticks. To refresh your memory of the other ways to select columns, see [select](#select).
 
 ```{r tidy-gather, echo = FALSE, out.width = "100%", fig.cap = "Gathering `table4` into a tidy form."}
 knitr::include_graphics("images/tidy-9.png")