docs: fix typo (#29553)

* Typo fix

* Update main.js

docs/fiddles/features/macos-dark-mode/main.js

@@ -22,7 +22,7 @@ function createWindow () {
   })
 
   ipcMain.handle('dark-mode:system', () => {
-    nativeTheme.themeSouce = 'system'
+    nativeTheme.themeSource = 'system'
   })
 }
 

docs/tutorial/dark-mode.md

@@ -159,7 +159,7 @@ function createWindow () {
   })
 
   ipcMain.handle('dark-mode:system', () => {
-    nativeTheme.themeSouce = 'system'
+    nativeTheme.themeSource = 'system'
   })
 }