doc: fix typo in readline completer function section

PR-URL: #48188
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zeyu "Alex" Yang <himself65@outlook.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>

doc/api/readline.md

@@ -744,7 +744,7 @@ function completer(line) {
 }
 ```
 
-The `completer` function can also returns a {Promise}, or be asynchronous:
+The `completer` function can also return a {Promise}, or be asynchronous:
 
 ```js
 async function completer(linePartial) {