docs(readme): point logo to v2.0.0 brand mark (sbpp#1349)

The README still embedded the v1.x logo from the v1.x branch
(.github/logo.png). Point it at the canonical 2.0.0 shield-with-cross
SVG that ships with the panel (web/themes/default/images/favicon.svg
— the source the docs site copies verbatim per docs/README.md).

Author: rumblefrog

README.md

@@ -1,5 +1,5 @@
 <h1 align="center">
-    <a href="https://sbpp.github.io"><img src="https://raw.githubusercontent.com/sbpp/sourcebans-pp/v1.x/.github/logo.png" height="25%" width="25%"/></a>
+    <a href="https://sbpp.github.io"><img src="https://raw.githubusercontent.com/sbpp/sourcebans-pp/main/web/themes/default/images/favicon.svg" height="25%" width="25%"/></a>
     <br/>
     SourceBans++
 </h1>