chore(docs) fix typo of the example code (#2315)

docs/usage/guides/visual/window-customization.md

@@ -82,7 +82,7 @@ document
   .addEventListener('click', () => appWindow.minimize())
 document
   .getElementById('titlebar-maximize')
-  .addEventListener('click', async () => await appWindow.isMaximized() ? appWindow.unmaximmize() : appWindow.maximize())
+  .addEventListener('click', async () => await appWindow.isMaximized() ? appWindow.unmaximize() : appWindow.maximize())
 document
   .getElementById('titlebar-close')
   .addEventListener('click', () => appWindow.close())