Fix typos in examples (#3714)

Author: yutannihilation

R/annotation-map.r

@@ -15,7 +15,7 @@ NULL
 #'
 #' seal.sub <- subset(seals, long > -130 & lat < 45 & lat > 40)
 #' ggplot(seal.sub, aes(x = long, y = lat)) +
-#'   annotation_map(usamap, fill = "NA", colour = "grey50") +
+#'   annotation_map(usamap, fill = NA, colour = "grey50") +
 #'   geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat))
 #' }
 #'
@@ -25,7 +25,7 @@ NULL
 #'   longr = cut(long, 2))
 #'
 #' ggplot(seal2,  aes(x = long, y = lat)) +
-#'   annotation_map(usamap, fill = "NA", colour = "grey50") +
+#'   annotation_map(usamap, fill = NA, colour = "grey50") +
 #'   geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat)) +
 #'   facet_grid(latr ~ longr, scales = "free", space = "free")
 #' }