fix(@angular/cli): ng update --help shows --packages twice (#12470)

* refactor: add a better `description` for `packages`

* fix(@angular/cli): `ng update --help` shows `--packages` twice

Closes #12468

Author: alan-agius4

packages/angular/cli/commands/update.json

@@ -11,22 +11,6 @@
 
   "type": "object",
   "allOf": [
-    {
-      "properties": {
-        "packages": {
-          "type": "array",
-          "items": {
-            "type": "string"
-          },
-          "description": "The names of package(s) to update",
-          "$default": {
-            "$source": "argv"
-          }
-        }
-      },
-      "required": [
-      ]
-    },
     {
       "$ref": "./definitions.json#/definitions/base"
     }

packages/schematics/update/update/schema.json

@@ -5,7 +5,7 @@
   "type": "object",
   "properties": {
     "packages": {
-      "description": "The packages to get.",
+      "description": "The names of package(s) to update.",
       "type": "array",
       "items": {
         "type": "string"