Fix typo in `AUTHORS` [ci skip]
## The List of Contributors sorted by number of commits (as of 2021-03-10)
7235 Yukihiro "Matz" Matsumoto (@matz)*
- 567 KOBAYASHI Shuji (@shuuji)
+ 567 KOBAYASHI Shuji (@shuujii)
376 Daniel Bovensiepen (@bovi)*
334 Masaki Muranaka (@monaka)
343 Takeshi Watanabe (@take-cheeze)*
