Add brief note explaining directories are folders

We use "directories" a lot on this page and it could be confusing for
someone used to calling them "folders".

Author: mxhold

sites/en/intro-to-rails/_switch_to_home_directory.step

@@ -1,5 +1,7 @@
 message "`cd` stands for change directory."
 
+message "A directory is the same thing as a folder."
+
 option "Windows" do
   console "cd c:\\Sites"
   message "`cd c:\\Sites` sets our Sites directory to our current directory."