Update protoc_plugin README install commands @V2

[zeroasterisk]

Flutter / Dart V2 no longer installs a pub binary. Now pub commands must be run through the main flutter or dart CLI binary.

[osa1]

Thanks @zeroasterisk for the PR and sorry for the late review.

We've updated the README in the meantime with some more instructions so this needs an update now.

Regarding the commands, I think for flutter we run the same commands, but using flutter instead of dart, right? E.g. instead of dart pub global activate ... we use flutter pub global activate .... So we could just say that Flutter users can just replace dart with flutter and that would be shorter. Any opinions on this?

[osa1]

So we could just say that Flutter users can just replace dart with flutter and that would be shorter.

If flutter install in a different path than dart that's another thing to mention.

[zeroasterisk]

I'm fine with your suggestion. You can cancel this PR and edit yourself, or I can pull and attempt a reword myself... The main correction I felt compelled to make, was to include a binary before the `pub`... That's the important bit.

[osa1]

OK, thanks.

We should mention that replacing dart with flutter also works, but users should follow https://docs.flutter.dev/get-started/install to update their PATH -- Flutter uses a different executable path than Dart.

I will do this in this PR if I have the permission to edit it, or create a new PR.