changing cmd to command as agreed

https://netlify.slack.com/archives/CGW2Y6XPH/p1553562219148000

Author: swyxio

README.md

@@ -91,7 +91,7 @@ The number of project types which Netlify Dev can detect is growing, but if your
 
 #sample dev block in the toml
 [dev]
-  cmd = "yarn start" # Command to start your dev server
+  command = "yarn start" # Command to start your dev server
   port = 3000 # Port that the dev server will be listening on
   publish = "dist" # Folder with the static content for _redirect file
 ```