Fix lambda function in multiindex_strategy to handle str dtypes

Signed-off-by: Glenn Sugar <[email protected]>

pandera/strategies/pandas_strategies.py

@@ -1192,7 +1192,7 @@ def multiindex_strategy(
         if dtype in {"object", "str"} or dtype.startswith("string"):
             # pylint: disable=cell-var-from-loop,undefined-loop-variable
             strategy = strategy.map(
-                lambda df: df.assign(**{name: df[name].map(str)})
+                lambda df, name=name: df.assign(**{name: df[name].map(str)})
             )
 
     if any(nullable_index.values()):