update reference to config file
`docker login` in 1.7 produces a config file in `~/docker/config.json`
instead of a `~/.dockercfg`.
@@ -20,7 +20,7 @@ do not specify a `SERVER`, the command uses Docker's public registry located at
You can log into any public or private repository for which you have
credentials. When you log in, the command stores encoded credentials in
-`$HOME/.dockercfg` on Linux or `%USERPROFILE%/.dockercfg` on Windows.
+`$HOME/.docker/config.json` on Linux or `%USERPROFILE%/.docker/config.json` on Windows.
# OPTIONS
**-e**, **--email**=""
