doc: fixed readable.isPaused() version annotation

readable.isPaused(): added a missing YAML placeholder
so the 'Added to version' annotation is displayed in docs

PR-URL: https://github.com/nodejs/node/pull/11677


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>