python: fix incorrect variable in pid resolver error message

Problem: An incorrect variable name is used in an error message
from the pid URI resolver when a flux-broker with no children is
detected. Therefore, instead of sensible error message, users see an
invalid name error.

Fix the error message to use the correct variable name so that a
sensible error is emitted.

Author: grondo

src/bindings/python/flux/uri/resolvers/pid.py

@@ -59,7 +59,7 @@ def _get_broker_child(pid):
                     return cpid
         except FileNotFoundError:
             pass
-    raise ValueError(f"PID {broker_pid} is a flux-broker and no child found")
+    raise ValueError(f"PID {pid} is a flux-broker and no child found")
 
 
 def _proc_has_task_children():