Fix documented Composer home variable name

Documentation fixes:

 - Fix documented Composer home variable name
 - Wrap long line
This is confusing. There is no other mention of `$home` (lowercase) in the documentation. Furthermore, [here](https://getcomposer.org/doc/03-cli.md#composer-cache-dir) `COMPOSER_CACHE_DIR` is documented as `$COMPOSER_HOME/cache`.

What is `$home`? If COMPOSER_HOME is not set, what value does `$home` have? Are you saying we have three types of "home" variable:

 - `$HOME` - User's home (`/home/<user>` on *nix)
 - `$COMPOSER_HOME` - Composer's home (by default `/home/<user>/.composer` on *nix)
 - `$home` - Not sure, some kind of internal variable?