Add missing US spellings for “scale_grey” (#4299)

Author: klmr

R/zxx.r

@@ -170,6 +170,16 @@ scale_color_stepsn <- scale_colour_stepsn
 #' @usage NULL
 scale_color_grey <- scale_colour_grey
 
+#' @export
+#' @rdname scale_grey
+#' @usage NULL
+scale_color_gray <- scale_colour_grey
+
+#' @export
+#' @rdname scale_grey
+#' @usage NULL
+scale_fill_gray <- scale_fill_grey
+
 #' @export
 #' @rdname scale_hue
 #' @usage NULL