pythongh-138295: Fix a grammar issue in the descriptor HOWTO (pythonGH-138296)

"an str" -> "a str"
(cherry picked from commit 552cf86)

Co-authored-by: Fangyi Zhou <[email protected]>

Author: fangyi-zhou

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}'