minor symfony#10162 [Workflow] Fix typo documentation (matthieumota)

This PR was submitted for the 4.1 branch but it was merged into the 3.4 branch instead (closes symfony#10162).

Discussion
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[Workflow] Fix typo documentation

In documentation, it indicate that ```audit_trail``` option wait a string, but it is an array who wait a ```enabled``` with boolean value.

Commits
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3d06c11 [Workflow] Fix typo documentation

Author: javiereguiluz

workflow/usage.rst

@@ -27,7 +27,8 @@ like this:
             workflows:
                 blog_publishing:
                     type: 'workflow' # or 'state_machine'
-                    audit_trail: 'enabled'
+                    audit_trail:
+                        enabled: true
                     marking_store:
                         type: 'multiple_state' # or 'single_state'
                         arguments:
@@ -63,7 +64,9 @@ like this:
         >
 
             <framework:config>
-                <framework:workflow name="blog_publishing" type="workflow" audit-trail="enabled">
+                <framework:workflow name="blog_publishing" type="workflow">
+                    <framework:audit_trail enabled="true" />
+
                     <framework:marking-store type="single_state">
                       <framework:argument>currentPlace</framework:argument>
                     </framework:marking-store>
@@ -107,7 +110,9 @@ like this:
             'workflows' => array(
                 'blog_publishing' => array(
                     'type' => 'workflow', // or 'state_machine'
-                    'audit_trail' => 'enabled',
+                    'audit_trail' => array(
+                        'enabled' => true
+                    ),
                     'marking_store' => array(
                         'type' => 'multiple_state', // or 'single_state'
                         'arguments' => array('currentPlace')
@@ -161,7 +166,7 @@ like this:
 
 .. tip::
 
-    Setting the ``audit_trail`` option to ``enabled`` makes the application
+    Setting the ``audit_trail.enabled`` option to ``true`` makes the application
     generate detailed log messages for the workflow activity.
 
     .. versionadded:: 3.3