feat: use URL for logo in README so it displays properly on PyPI.

Original code used a relative path to the logo file in `README.md`,
but that does not resolve properly when the package is uploaded to
pypi.org.  This PR replaces the relative path with the full GitHub URL
to the logo file, which is clunky but works.

Author: gvwilson

README.md

@@ -1,6 +1,6 @@
 <p align="center">
   <a href="https://dash-bootstrap-components.opensource.faculty.ai/">
-    <img src="./readme-images/logo.png" alt="dash-bootstrap-components logo" width="200" height="200">
+    <img src="https://github.com/facultyai/dash-bootstrap-components/blob/main/readme-images/logo.png" alt="dash-bootstrap-components logo" width="200" height="200">
   </a>
 </p>