Learning DVCS is important; git is just one tool.

While git is far-and-wide the most-adopted DVCS in the ruby
and modern open-source worlds, it is just one tool and there
will be others to replace it. This commit changes wording in
order to pose the need for DVCS and Git as a modern-and-valid
solution.

Incorporates [feedback][] from @kerrizor

[feedback]: https://github.com/Ada-Developers-Academy/curriculum/pull/7#discussion_r6671506
Don't conflate "distributed" in DVCS with remote servers.

SVN is an example of a non-distributed VCS, and the given example
applies to it as well. This commit attempts to clarify that the
"distributed" attribute allows us to do work *without* the
servers, in addition to being able to push & pull code.

Incorporates [clarification] suggested by @meonkeys.

[clarification]: https://github.com/Ada-Developers-Academy/curriculum/pull/7#discussion_r6671645
Share MD links to Git Immersion.
Merge pull request #7 from yaauie/01-git-improvements