Change structure of error message to correct grammar (#3775)

* fixes grammar in error message

Author: wendtke

R/theme-elements.r

@@ -524,7 +524,7 @@ validate_element <- function(el, elname, element_tree) {
     if (!is.unit(el) && length(el) == 4)
       abort(glue("Theme element `{elname}` must be a unit vector of length 4."))
   } else if (!inherits(el, eldef$class) && !inherits(el, "element_blank")) {
-      abort(glue("Theme element `{elname}` must be an `{eldef$class}` object."))
+      abort(glue("Theme element `{elname}` must be an object of type `{eldef$class}`."))
   }
   invisible()
 }