Update README.md

Author: mbostock

README.md

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 # Observable Plot
 
-[<img src="./img/plot.svg" width="320" alt="The Observable Plot logo, spelling out the letters P-L-O-T in pastel shapes.">](https://observablehq.com/plot/)
+[<img src="https://observablehq.com/plot/plot.svg" width="320" alt="The Observable Plot logo, spelling out the letters P-L-O-T in pastel shapes.">](https://observablehq.com/plot/)
 
 [**Observable Plot**](https://observablehq.com/plot/) is a free, [open-source](./LICENSE), JavaScript library for visualizing tabular data, focused on accelerating exploratory data analysis. It has a concise, memorable, yet expressive API, featuring [scales](https://observablehq.com/plot/features/scales) and [layered marks](https://observablehq.com/plot/features/marks) in the *grammar of graphics* style.