Fixing `source` typo (#27583)

docs/tutorial/dark-mode.md

@@ -154,7 +154,7 @@ function createWindow () {
   })
 
   ipcMain.handle('dark-mode:system', () => {
-    nativeTheme.themeSouce = 'system'
+    nativeTheme.themeSource = 'system'
   })
 }