chore: minor fix for DebugLoggingPlugin example

Co-authored-by: Xuan Yang <xygoogle@google.com>
PiperOrigin-RevId: 859762221

Author: xuanyang15

src/google/adk/plugins/debug_logging_plugin.py

@@ -82,7 +82,7 @@ class DebugLoggingPlugin(BasePlugin):
   Example:
       >>> debug_plugin = DebugLoggingPlugin(output_path="/tmp/adk_debug.yaml")
       >>> runner = Runner(
-      ...     agents=[my_agent],
+      ...     agent=my_agent,
       ...     plugins=[debug_plugin],
       ... )