Bump acorn from 6.1.1 to 6.4.1 in /books

dependabot[bot]

2020-04-04 d76796e2c1baa698ef40f9ccd764b9878ff3d9e0

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s/listen       80/listen       8080/
s/listen       \[::\]:80/listen       \[::\]:8080/
s/user nginx;//
s%error_log /var/log/nginx/error.log%error_log stderr%
s%access_log  /var/log/nginx/access.log  main;%%