fix(README): 改正写错的背景图配置项

Author: Alex6357

src/README.md

@@ -7,6 +7,6 @@ heroImage: /assets/logo.png
 heroText: PCService 志愿者服务队
 heroFullScreen: true
 tagline: 拯救全校师生的生产力危机
-bzImage: /assets/background.png
+bgImage: /assets/background.png
 bgImageDark: /assets/background.png
 ---