Updated wording on donate box after consulting with LF

[jasongrout]

LF suggested several wording changes to make things more clear and accurate in our donation box.

[choldgraf]

@jasongrout can you provide guidance for sub-projects that also have donate boxes? E.g. the donate button on the mybinder.org support page points to numfocus but probably needs to be updated.

[jasongrout]

@choldgraf: Great question. We just updated the main jupyter readthedocs to point to https://jupyter.org/about#donate so that there is one place we have to change in the future: jupyter/jupyter#746 (rendered at https://docs.jupyter.org/en/latest/#resources)

I personally would suggest any donation links in the project should point to https://jupyter.org/about#donate for now.

(...hey, nothing that one more level of indirection can't solve :)