Docs: Fix formatting in table (#11016)

Fixes formatting in a table cell in `logstash-monitoring-overview.html`.
A `+` which was required by AsciiDoc was leaking into the output when
the doc is built with Asciidoctor.

Author: nik9000

docs/static/monitoring/collectors.asciidoc

@@ -16,7 +16,7 @@ collectors: one for node stats and one for pipeline stats.
 | Node Stats      | `logstash_stats`
 | Gathers details about the running node, such as memory utilization and CPU
 usage (for example, `GET /_stats`).
-+
+
 This runs on every Logstash node with {monitoring} enabled. One common
 failure is that Logstash directories are copied with their `path.data` directory
 included (`./data` by default), which copies the persistent UUID of the Logstash