FreeBSD wget cannot verify certificate, issued by Let's Encrypt

Originally published at: https://www.cyberciti.biz/faq/freebsd-wget-cannot-verify-certificate-issued-by-lets-encrypt/

I installed GNU wget utility on FreeBSD as explained here. However, whenever I use the wget command to download stuff from the Internet, it says:
   ERROR: cannot verify download.freebsd.org’s certificate, issued by ‘CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US’:
    Unable to locally verify the issuer’s authority.

How do I fix this problem on FreeBSD 12?