Changed `ruby /path/to/rails myapp --dev` to `ruby /path/to/rails new myapp...

Changed `ruby /path/to/rails myapp --dev` to `ruby /path/to/rails new myapp --dev` in the "Thor is not avalable" message. [#4915 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

railties/lib/rails/generators/base.rb

@@ -3,7 +3,7 @@
 rescue LoadError
   puts "Thor is not available.\nIf you ran this command from a git checkout " \
        "of Rails, please make sure thor is installed,\nand run this command " \
-       "as `ruby /path/to/rails myapp --dev`"
+       "as `ruby /path/to/rails new myapp --dev`"
   exit
 end