Update 03-programming.Rmd (#288)

Change variable name to be more consistent.

03-programming.Rmd

@@ -425,7 +425,7 @@ row_sd = apply(ex_mat, 1, sd)
 The first argument of `apply()` is the object of interest. The second argument is the `MARGIN`. This is a vector giving the subscripts which the function (the third argument) will be applied over. When the object is a matrix, a margin of `1` indicates rows and `2` indicates columns.  So to calculate the column standard deviations, the second argument is changed to `2` 
 
 ```{r, results="hide"}
-col_med = apply(ex_mat, 2, sd)
+col_sd = apply(ex_mat, 2, sd)
 ```
 
 Additional arguments can be passed to the function that is to be applied to the data. For example, to pass the `na.rm` argument to the `sd` function, we have