Help: Fix typo in 3.31.9 release note heading

Help/release/3.31.rst

@@ -267,7 +267,7 @@ Updates
 
 Changes made since CMake 3.31.0 include the following.
 
-3.31.1, 3.31.2, 3.31.3, 3.31.4, 3.31.5, 3.31.6, 3.31.7, 3.31.8, 3,31.9
+3.31.1, 3.31.2, 3.31.3, 3.31.4, 3.31.5, 3.31.6, 3.31.7, 3.31.8, 3.31.9
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 * These versions made no changes to documented features or interfaces.