Add missing value for page-break-before (#741)

Author: vuejs-jp-bot

themes/vue/source/css/_common.styl

@@ -226,7 +226,7 @@ sup.beta.beta
   h1,h2,h3,h4,h5
     color: rgb(0,0,0)
     page-break-after: avoid
-    page-break-before: 
+    page-break-before: avoid
   img
     page-break-inside: avoid
   h2+p