added new example using mutate() (#428)

to further clarify how to use replace_na() in data frames. from #427

Author: foundinblank

R/replace_na.R

@@ -10,6 +10,7 @@
 #' library(dplyr)
 #' df <- tibble(x = c(1, 2, NA), y = c("a", NA, "b"), z = list(1:5, NULL, 10:20))
 #' df %>% replace_na(list(x = 0, y = "unknown"))
+#' df %>% mutate(x = replace_na(x, 0))
 #'
 #' # NULL are the list-col equivalent of NAs
 #' df %>% replace_na(list(z = list(5)))