update StartupWMClass

this seems to have changed in the latest version

Author: DetachHead

app.desktop

@@ -7,7 +7,7 @@ Comment=Code Editing. Redefined.
 Exec=code --unity-launch %F
 Icon=code
 Categories=TextEditor;Development;IDE;
-StartupWMClass=Code
+StartupWMClass=code
 StartupNotify=true
 Keywords=vscode;Visual Studio Code;Code;
 MimeType=text/plain;inode/directory;application/x-code-workspace;