Guide: filename mismatch in Section 2 Hello, world!

in guide.md section 2
code example should refer either to main OR hello_word 
but mixing them in a single example make no sense i.e:

$ rustc main.rs
$ ./hello_world # or hello_world.exe on Windows

either use the prev chosen name
$ ./main # or main.exe on Windows
OR
$ rustc hello_world.rs
(and change the prev chosen file name)

same mistake is repeated all over the section
