fix typo in changelog

Author: SkySkimmer

doc/sphinx/changes.rst

@@ -298,7 +298,7 @@ Tactics
   (`#17745 <https://github.com/coq/coq/pull/17745>`_,
   by Gaëtan Gilbert).
 - **Changed:**
-  instances declared with flag:`Typeclasses Unique Instances` do not allow backtracking even when the goal contains evars
+  instances declared with :flag:`Typeclasses Unique Instances` do not allow backtracking even when the goal contains evars
   (`#17789 <https://github.com/coq/coq/pull/17789>`_,
   fixes `#6714 <https://github.com/coq/coq/issues/6714>`_,
   by Jan-Oliver Kaiser).