refactor: fix lint warning on lib/letter-avatars.js
refactor: fix lint warning on lib/letter-avatars.js

Signed-off-by: BoHong Li <9f7d63b58e4573d72eb50da9dd011b77faf5d27f@gmail.com>

jackycute/HackMD,hackmdio/hackmd,hackmdio/hackmd,jackycute/HackMD,hackmdio/hackmd,jackycute/HackMD