fix irregular radius on code samples (#1256)

Author: mychidarko

src/.vuepress/theme/styles/code.styl

@@ -142,7 +142,7 @@ div.reactivecontent
 
 // @import '~prismjs/themes/prism-tomorrow.css'
 
-.token.punctuation 
+.token.punctuation
   color #a8a9cc
 
 .token.tag, .token.attr-name, .token.namespace, .token.deleted
@@ -169,4 +169,6 @@ div.reactivecontent
 div[class*=language-].line-numbers-mode .line-numbers-wrapper, div[class*=language-].line-numbers-mode:after
   background-color #f6f6f6
   color #c4c4c6
-  border-right 1px solid #e7e6e6
+  border-right 1px solid #e7e6e6
+  border-top-left-radius 6px
+  border-bottom-left-radius 6px