doc: use kbd element in readline doc prose

The kbd element has already been added in the keybindings table, but we
missed it in one line in the prose.

PR-URL: #35737
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

doc/api/readline.md

@@ -77,7 +77,7 @@ added: v0.1.98
 
 The `'line'` event is emitted whenever the `input` stream receives an
 end-of-line input (`\n`, `\r`, or `\r\n`). This usually occurs when the user
-presses the `<Enter>`, or `<Return>` keys.
+presses <kbd>Enter</kbd> or <kbd>Return</kbd>.
 
 The listener function is called with a string containing the single line of
 received input.