Merge pull request elixir-lang#715 from Steve-O-Cassels/patch-1

Fix windows set environment cmd

Author: josevalim

getting-started/mix-otp/introduction-to-mix.markdown

@@ -255,7 +255,7 @@ $ MIX_ENV=prod mix compile
 Or on Windows:
 
 ```batch
-> set /a "MIX_ENV=prod" && mix compile
+> set "MIX_ENV=prod" && mix compile
 ```
 
 ## Exploring