check-format.pl: Rename 'one-letter' to 'single-letter', do not report 'l'
check-format.pl: Rename 'one-letter' to 'single-letter', do not report 'l'

Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15077)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl