Update examples/ustack.md

Co-authored-by: sourcery-ai[bot] <58596630+sourcery-ai[bot]@users.noreply.github.com>

examples/ustack.md

@@ -132,7 +132,7 @@ You can use `copy()` method for copying `Stack`s:
 s.copy()
 ```
 
-You can also use `copy.copy()` function for copying `Stack`s:
+You can use the `copy.copy()` function to copy `Stack`s:
 ```python
 from copy import copy