Update 02-variables.md upper-lower case change

[teresa-m]

Thanks a lot for the great learning materials. I have one suggestion. Within the Tutorial Plotting and Programming in Python in the Variable and Assignment part, I would change the sentence about upper and lower case naming conventions for variables. The original statement 'upper-case letters are the conventions ... so we use lower case' is ambiguous. I guess this was just mixed up, but if not this would need further explanation in my opinion.

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[vahtras]

Thanks for raising the question. However, I don't think the suggested change makes anything clearer. There are a number of conventions and style recommendations which may mean going down a rabbit hole. Any alternative formulation, anyone?