fixed wrong caption level for plugin documentation (#894)

Co-authored-by: Johannes Erwerle <[email protected]>

docs/plugins/index.rst

@@ -78,7 +78,7 @@ A transform function is a plugin that manipulates the inventory independently fr
 During inventory initialization, the transform function will be called in a for loop for each host. The transform function takes a host object as the first parameter and additional keyword arguments as specified in the `config.inventory.transform_function_options` dictionary.
 
 Runners
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 A runner is a plugin that dictates how to execute the tasks over the hosts