Merge pull request #67 from nakashima-hikaru/patch-1

docs: Fix example usage

README.md

@@ -49,7 +49,7 @@ Patito allows you to specify the type of each column in your dataframe by creati
 
 ```py
 # models.py
-from typing import Literal, Optional
+from typing import Literal
 
 import patito as pt
 
@@ -74,7 +74,7 @@ df = pl.DataFrame(
 )
 try:
     Product.validate(df)
-except pt.ValidationError as exc:
+except pt.exceptions.DataFrameValidationError as exc:
     print(exc)
 # 3 validation errors for Product
 # is_for_sale
@@ -120,7 +120,7 @@ def num_products_for_sale(products: pl.DataFrame) -> int:
     return products.filter(pl.col("is_for_sale")).height
 ```
 
-The following test would fail with a `patito.ValidationError`:
+The following test would fail with a `patito.exceptions.DataFrameValidationError`:
 
 ```py
 def test_num_products_for_sale():