Fix URL to robots.txt documentation

guides/code/getting_started/public/robots.txt

-# See http://www.robotstxt.org/wc/norobots.html for documentation on how to use the robots.txt file
+# See http://www.robotstxt.org/robotstxt.html for documentation on how to use the robots.txt file
 #
 # To ban all spiders from the entire site uncomment the next two lines:
 # User-agent: *

railties/lib/rails/generators/rails/app/templates/public/robots.txt

-# See http://www.robotstxt.org/wc/norobots.html for documentation on how to use the robots.txt file
+# See http://www.robotstxt.org/robotstxt.html for documentation on how to use the robots.txt file
 #
 # To ban all spiders from the entire site uncomment the next two lines:
 # User-agent: *