DOCS-13914 update 4.0.21 summary ticket description

Author: andf-mongodb

source/release-notes/4.0.txt

@@ -37,7 +37,7 @@ Issues fixed:
   self-ownership
 - :issue:`SERVER-50915` [v4.0] fsyncLock must not take a stable
   checkpoint when majority read concern is off
-- :issue:`SERVER-51120` Find queries with MERGE_SORT incorrectly sort
+- :issue:`SERVER-51120` Find queries with SORT_MERGE incorrectly sort
   the results when the collation is specified
 
 - `All JIRA issues closed in 4.0.21