Ubuntu: nGinx – Fix “The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY ABF5BD827BD9BF62”

To fix the following issue, type the following commands in a terminal:
$ wget http://nginx.org/packages/keys/nginx_signing.key
$ cat nginx_signing.key | sudo apt-key add -
OK
$ apt-get update
$ apt-get install nginx