website: fix English book sidebar heading (was 'Book-en-us')

The sidebar section heading matched only book-zh-cn, so English chapters
fell through to the raw-type fallback and showed 'Book-en-us'. Match
book-en-us too so it reads 'Main', mirroring the Chinese '正文'.

Author: changkun

website/themes/moderncpp/layout/partials/sidebar.ejs

@@ -14,7 +14,7 @@
           <%-
           type === 'exercises'
               ? 'Exercises'
-              : type === 'book-zh-cn'
+              : type === 'book-en-us' || type === 'book-zh-cn'
               ? 'Main'
               : type === 'answers'
                   ? 'Reference Answers'