add absolute image link, so that it work in pypi

Author: HugoMVale

README.md

@@ -15,7 +15,7 @@ the explanatory and response variables have significant errors.
 
 <p align="center">
   <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/81/Total_least_squares.svg/220px-Total_least_squares.svg.png" width="250" alt="Deming regression; special case of ODR." style="margin-right: 10px;">
-  <img src="docs/examples/ellipse.svg" width="400" alt="Estimated ellipse parameters.">
+  <img src="https://github.com/HugoMVale/odrpack-python/blob/main/docs/examples/ellipse.svg" width="400" alt="Estimated ellipse parameters.">
 </p>
 
 [errors-in-variables regression]: https://en.wikipedia.org/wiki/Errors-in-variables_models