gh-138295: Fix a grammar issue in the descriptor HOWTO (GH-138296)

"an str" -> "a str"

diff --git a/Doc/howto/descriptor.rst b/Doc/howto/descriptor.rst
index f6c3e473f1c36d3f188c721bf0da160d4e4de785..9d5a9ac8b718cb00e55567e7d544ab82f130a7c2 100644 (file)
--- a/Doc/howto/descriptor.rst
+++ b/Doc/howto/descriptor.rst
@@ -420,7 +420,7 @@ Here are three practical data validation utilities:
 
         def validate(self, value):
             if not isinstance(value, str):
-                raise TypeError(f'Expected {value!r} to be an str')
+                raise TypeError(f'Expected {value!r} to be a str')
             if self.minsize is not None and len(value) < self.minsize:
                 raise ValueError(
                     f'Expected {value!r} to be no smaller than {self.minsize!r}'