tweak: fix padding for highlighted line (#149)

Author: egoist

lib/default-theme/styles/code.styl

@@ -35,7 +35,7 @@
     background-color rgba(0, 0, 0, 66%)
     display block
     margin 0.1rem -1.5rem 0
-    padding 0.1rem 1.8rem
+    padding 0.1rem 1.5rem
 
 pre[class="language-js"], pre[class="language-javascript"]
   &:before