doc: fix Markdown syntax in ops.testing readme

The previous syntax made Markdown treat the entire paragraph as the title. I don't think this was intended. Maybe there was originally a list or something else at this point, but this changes the formatting back to a normal title for a single sentence, and then the following paragraph is formatted normally.

Copied from @jameinel 's PR in the old repo: canonical/ops-scenario#205

(I accidentally made this change in 0c55e64).

testing/README.md

@@ -613,7 +613,7 @@ Pebble can generate notices, which Juju will detect, and wake up the charm to
 let it know that something has happened in the container. The most common
 use-case is Pebble custom notices, which is a mechanism for the workload
 application to trigger a charm event.
--
+
 When the charm is notified, there might be a queue of existing notices, or just
 the one that has triggered the event: