fix use of weights in fitting tutorial

[keatonb]

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I changed all instances of weights = 1.0/error**2 to weights = 1.0/error to address #570.

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[rkiman]

Thank you for catching this! I would just modify your note "Note that the fitting routine takes weights as 1/error and squares them for you, which is different from the definition of weights used on the linked pages." to: Note that the fitting routine takes weights as 1/error and squares them for you, as indicated in the description of the function (and add the link, even though it is indicated in other parts of the tutorial).

[keatonb]

I made the change you suggested @rkiman. Cheers!

[adrn]

Thanks @keatonb and @rkiman !