Readme: update logo filename

Commit 0dd6ac1 ("gui: rename logo's
filename to icon name") caused the filename of the logo to change. All
instances of the filename were also renamed, besides in Readme.md.

Fix it here too.

Fixes: 0dd6ac1 ("gui: rename logo's filename to icon name")
Reported-by: @ozmodeuz

Readme.md

@@ -1,4 +1,4 @@
-<img src="https://raw.githubusercontent.com/quexten/goldwarden/main/gui/goldwarden.svg" width=200>
+<img src="https://raw.githubusercontent.com/quexten/goldwarden/main/gui/com.quexten.Goldwarden.svg" width=200>
 
 # Goldwarden