Encode @ to fix bungled docs

Fixes #4

It looks like the context free parser is stripping out this line because it has an `@import` in the comment.

core-style.html

@@ -18,7 +18,7 @@
 multiple places, for example, in many different elements.
 
 It's common to place `core-style` producer elements inside HTMLImports.
-Remote stylesheets should be included this way, the @import css mechanism is
+Remote stylesheets should be included this way, the @import css mechanism is
 not currently supported.
 
 Here's a basic example: