Update Changelog.rst

Co-authored-by: Sadie L. Bartholomew <[email protected]>

Author: davidhassell

Changelog.rst

@@ -9,7 +9,7 @@ Version NEXTVERSION
 * Allow multiple conditions for the same axis in `cf.Field.subspace`
   and `cf.Field.indices`
   (https://github.com/NCAS-CMS/cf-python/issues/881)
-* Fix bug in `cf.Field.collapse` that causes an Exception to be raised
+* Fix bug in `cf.Field.collapse` that causes a ``ValueError`` to be raised
   for missing external cell measures data
   (https://github.com/NCAS-CMS/cf-python/issues/885)
 * New dependency: ``distributed>=2025.5.1``