Update index.md. Fix 404 getcomposer.org link.
The link to getcomposer in the documentation is 404. It has a "www" prefix, but the upstream site doesn't use it. The correct link is simply https://getcomposer.org/
This fixes the link on this page.
The link to getcomposer in the documentation is 404. It has a "www" prefix, but the upstream site doesn't use it. The correct link is simply https://getcomposer.org/
This fixes the link on this page.