docs: fix typing example

docs/api-reference/typing.md

@@ -33,9 +33,9 @@ from narwhals.typing import IntoDataFrameT, DataFrameT
 df = pl.DataFrame({"a": [1, 2, 3]})
 
 
-def func(df: IntoDataFrameT) -> IntoDataFrameT:
-    df = nw.from_native(df, eager_only=True)
-    return nw.to_native(df.select(b=nw.col("a")))
+def func(df_native: IntoDataFrameT) -> IntoDataFrameT:
+    df = nw.from_native(df_native, eager_only=True)
+    return df.select(b=nw.col("a")).to_native()
 
 
 reveal_type(func(df))
@@ -51,9 +51,9 @@ reveal_type(func_2(df))
 
 Running `mypy` on it gives:
 ```console
-$ mypy f.py 
-f.py:11: note: Revealed type is "polars.dataframe.frame.DataFrame"
-f.py:17: note: Revealed type is "Any"
+$ mypy t.py 
+t.py:13: note: Revealed type is "polars.dataframe.frame.DataFrame"
+t.py:21: note: Revealed type is "Any"
 Success: no issues found in 1 source file
 ```