Fix Pelicun logo link

The pelicun logo was originally pulled from the gh-pages branch but with
the new docs it is no longer available at that location. Instead, we can
have it sourced from the master branch which seems much more appropriate

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-	<img src="https://raw.githubusercontent.com/NHERI-SimCenter/pelicun/gh-pages/doc_src/common/figures/pelicun-Logo-white.png"
+	<img src="https://raw.githubusercontent.com/NHERI-SimCenter/pelicun/master/doc/source/_static/pelicun-Logo-white.png"
 		alt="pelicun" align="middle" height="200"/>
 </p>