switch to lowercase 'plotly' require.js dependency

[marius311]

Plots.jl makes the RequiresJS dependency uppercase 'Plotly' but plotly.py and PlotlyJS have it lowercase 'plotly'. If you export figures from each of them and try to splice them into the same parent document, it makes it really hard to resolve the dependency (since to my very noob knowledge, you can't load the same library under two different names). This PR makes Plots.jl consistent with the others.

Have not tested extensively beyond my simple cases so someone that knows what they're doing should definitely verify this.

[codecov]

Codecov Report

Base: 90.80% // Head: 90.80% // No change to project coverage 👍

Coverage data is based on head (6da30da) compared to base (2eb315f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4640   +/-   ##
=======================================
  Coverage   90.80%   90.80%           
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  Files          41       41           
  Lines        8778     8778           
=======================================
  Hits         7971     7971           
  Misses        807      807           

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