Update preinstalled plugins list (#1451)

Co-authored-by: GitHub Actions <github-actions@github.com>

Author: jmagak

modules/dynamic-plugins/con-preinstalled-dynamic-plugins.adoc

@@ -11,6 +11,7 @@ The following preinstalled dynamic plugins are enabled by default:
 * `@backstage-community/plugin-analytics-provider-segment`
 * `@backstage-community/plugin-scaffolder-backend-module-quay`
 * `@backstage-community/plugin-scaffolder-backend-module-regex`
+* `@backstage/plugin-events-backend`
 * `@backstage/plugin-techdocs`
 * `@backstage/plugin-techdocs-backend`
 * `@backstage/plugin-techdocs-module-addons-contrib`