Fix readme git package usage

[nickmeinhold]

Hello and thank you for your work!

When I follow the instructions in the README on adding the dependency as a Git package, I get the following error:

Resolving dependencies...
Could not find a file named "pubspec.yaml" in git://github.com/pubnub/dart.git 01a375fc5f1ad744fbd3df92aa9c8b6d5ab74fed.
pub finished with exit code 1

I have updated the README with what I believe is the correct usage in this case.

Thanks!

[are]

Hi! Thanks for opening the PR! You are absolutely correct, that part of README needed to be updated! We will incorporate your changes in the next release.