ruff: Fix PLR6104 Use += to perform an augmented assignment directly.

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Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/markdown/api_arguments_table_generator.py

@@ -307,5 +307,5 @@ def generate_data_type(schema: Mapping[str, Any]) -> str:
     else:
         data_type = schema["type"]
         if schema.get("nullable", False):
-            data_type = data_type + " | null"
+            data_type += " | null"
     return data_type