fix: set the font-weight for strong tags back to 600 (#1048)

Author: skirtles-code

src/.vuepress/theme/styles/index.styl

@@ -115,7 +115,7 @@ ul, ol
   padding-left 1.2em
 
 strong
-  font-weight 500
+  font-weight 600
 
 h1, h2, h3, h4, h5, h6
   font-weight 500