Fix example to use correct method of Stack (python#19123)

This PR updates the `generics.rst` documentation to correct a method
call in the `Stack` usage example. Previously, the example incorrectly
used `.append('x')` on a `Stack[str]` instance, which is not a valid
method for the `Stack` class.

Author: Kwieeciol

docs/source/generics.rst

@@ -93,7 +93,7 @@ Using ``Stack`` is similar to built-in container types:
    stack.push('x')
 
    stack2: Stack[str] = Stack()
-   stack2.append('x')
+   stack2.push('x')
 
 Construction of instances of generic types is type checked (Python 3.12 syntax):