Fix some typos in contributing / documentation

[Pierre Poulain]

The Contributing part of the documentation explains how to install plasma locally.

To pull the the repo2docker Docker image, the command

docker pull jupyter/repo2docker

failed:

Using default tag: latest
Error response from daemon: manifest for jupyter/repo2docker:latest not found: manifest unknown: manifest unknown

and should be replaced by:

docker pull jupyter/repo2docker:master

as in tljh-repo2docker.

[Pierre Poulain]

I propose to update the documentation accordingly.

[Jeremy Tuloup]

Good catch.

The fix is indeed to change:

docker pull jupyter/repo2docker

By:

docker pull jupyter/repo2docker:master

Here: https://github.com/plasmabio/plasma/blob/master/docs/contributing/local.rst#pull-the-repo2docker-docker-image