[doc]Composer is not a package manager.

It is a issue i found when I translate it.
> Composer is not a package manager, It's a tool for dependency management.

According to their [doc](https://getcomposer.org/doc/00-intro.md)
>Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.

and 

> **Composer is not a package manager.** Yes, it deals with "packages" or libraries, but it manages > them on a per-project basis, installing them in a directory (e.g. vendor) inside your project. By > default it will never install anything globally. Thus, it is a dependency manager.

So i think, It will be better, if we remain their offical appellation.
Merge pull request #2718 from qiansen1386/patch-2

[doc]Composer is not a package manager.