[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

keyserver/addons/rust-node-addon/src/identity_client/add_reserved_usernames.rs

@@ -2,7 +2,7 @@ use super::*;
 
 #[napi]
 #[instrument(skip_all)]
-pub async fn add_reserved_username(
+pub async fn add_reserved_usernames(
   message: String,
   signature: String,
 ) -> Result<()> {