Correct rails and rake command syntaxes in the /app page.

I've checked this changes with follow environments. (according to /install page).

[Windows] RailsInstaller, Ruby1.9.3p392/Rails4.1.0, Win 8.1
"rails" command use "bin/rails". (Using binstub and spring)
"rake" command don't use "bin/rake", so we need "ruby bin/rake" command. ( "/" is correct in RailsInstaller shell. )

[Mac] rbenv Ruby2.1.2/Rails4.1.0
"rails" command use "bin/rails". (Using binstub and spring)
"rake" command don't use "bin/rake", so we need "bin/rake" command.