[keyserver] rename napi add_reserved_usernames function

Summary:
accidentally renamed addReservedUsername -> addReservedUsernames in JS without making the corresponding change to the rust function name.

i think i missed this when i split out D8300 into multiple diffs. should've tested more thoroughly after splitting and rebasing

Test Plan:
Registered a new user and saw the new username (albert) in my reserved usernames DDB table

Also deleted albert's account and saw that the username was removed from the table

2023-07-18T19:29:13.715822Z INFO identity::database: Batch write item to reserved usernames table succeeded
2023-07-18T19:30:51.876524Z INFO identity::database: Username albert has been deleted from reserved usernames table

Reviewers: ashoat, jon

Reviewed By: jon

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8533