fix($theme-default): fit image for hero image (#980)

Author: Vladislav-Lyuminarskiy

packages/@vuepress/theme-default/components/Home.vue

@@ -78,6 +78,7 @@ export default {
   .hero
     text-align center
     img
+      max-width: 100%
       max-height 280px
       display block
       margin 3rem auto 1.5rem