Apache can only take care of the server side of things. I.e., it can internally rewrite /aboutus to /code/aboutus.html.
You still have to update the link in your HTML yourself.
Change
<a href=“/code/aboutus.html”>About us</a> to <a href=“/aboutus”>About us</a>
I would also like to suggest some slight changes to your .htaccess
RewriteEngine On
RewriteRule ^/?aboutus/?$ /code/aboutus.html [L]
Remove [NC], not needed (only need when checking information that are case sensitive, folders are not)
Add /? at the beginning of the RewriteRule, to make it compatible with Apache 1.x might you ever need it
Prepended a / to code/aboutus.html, to make it more clear where it is, and prevent possible problems with this later