Fix blog feed escaping of unicode characters

Summary:
The pattern correctly matches the unicode characters, but
should not be saving the u character in the group. Otherwise the output
is something like &#xu00e9; which is invalid and ends up rendering as
those plain characters rather than the expected é which is é

Test Plan:
Remove the .slice(0, 3) from js/blog-feed.js to see more articles
if the recent ones do not display the above issue.
make serve
Navigate to localhost:4000 and see the feed rendering unicode characters correctly.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7465