Correct guide generation command [ci skip]

guides/rails_guides/generator.rb

@@ -28,12 +28,12 @@
 #     enough:
 #
 #       # generates only association_basics.html
-#       ONLY=assoc ruby rails_guides.rb
+#       ONLY=assoc rake guides:generate
 #
 #     Separate many using commas:
 #
-#       # generates only association_basics.html and migrations.html
-#       ONLY=assoc,migrations ruby rails_guides.rb
+#       # generates only association_basics.html and command_line.html
+#       ONLY=assoc,command rake guides:generate
 #
 #     Note that if you are working on a guide generation will by default process
 #     only that one, so ONLY is rarely used nowadays.