Get list of template variables for xtriggers from correct place

from cylc.flow.xtrigger_mgr.TemplateVariables, not
task_events_mgr.EventData

Author: wxtim

src/user-guide/writing-workflows/external-triggers.rst

@@ -254,7 +254,11 @@ properties:
    because each call is executed in an independent process in the process
    pool. If necessary the filesystem can be used for this purpose.
 
-.. autoenumvalues:: cylc.flow.task_events_mgr.EventData
+.. spelling:word-list::
+
+   vv
+
+.. autoenumvalues:: cylc.flow.xtrigger_mgr.TemplateVariables
 
 Function return values should be as follows: