Remove "project" from log path in callback docs (apache#42666)

Airflow doesn't have the concept of a "project", unless DAG authors add
that layer themselves.

docs/apache-airflow/administration-and-deployment/logging-monitoring/callbacks.rst

@@ -34,7 +34,7 @@ For example, you may wish to alert when certain tasks have failed, or have the l
     Callback functions are executed after tasks are completed.
     Errors in callback functions will show up in scheduler logs rather than task logs.
     By default, scheduler logs do not show up in the UI and instead can be found in
-    ``$AIRFLOW_HOME/logs/scheduler/latest/PROJECT/DAG_FILE.py.log``
+    ``$AIRFLOW_HOME/logs/scheduler/latest/DAG_FILE.py.log``
 
 Callback Types
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