minor fixes to contributing instructions

[jwhendy]

Super minor changes from reading your contributing.md instructions as a new, would-be-contributor.

The instructed url to clone, https://github.com/plotly/python-api redirects here, so it seemed like one less thing to be confused about, especially since the instructions seemed geared toward being noob friendly.

Also, since you're instructing to fork, I changed your clone url:

git clone https://github.com/your_github_username/plotly.py.git

I've done a reasonable amount of PRs and still get burned more than I'd like by cloning the upstream and then realizing I can't push my changes like I thought because I'm not in my fork!

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Aside: maybe you meant for users to simply clone this repo, but then I'd suggest some different changes to clarify that since it's right after telling them to learn how to fork.

[jackparmer]

👍