[3.10] [docs] Improve the markup of powers (GH-28598) (GH-28607)

(cherry picked from commit 4f05f15d7b25ef8b690cb94fdc4c8cb5521a4e27)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 3713f04..310167e 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1523,7 +1523,7 @@
 
       This is intended to provide protection against a denial-of-service caused
       by carefully-chosen inputs that exploit the worst case performance of a
-      dict insertion, O(n^2) complexity.  See
+      dict insertion, O(n\ :sup:`2`) complexity.  See
       http://www.ocert.org/advisories/ocert-2011-003.html for details.
 
       Changing hash values affects the iteration order of sets.