27.1: fix formatting in release notes

Not completely sure what the issue is, I guess we can't use the word
Index, however this fixes the formatting of the release notes:

Author: fanquake

_releases/27.1.md

@@ -77,7 +77,7 @@ Notable changes
 - #29870 rpc: Reword SighashFromStr error message
 - #30094 rpc: move UniValue in blockToJSON
 
-### Index
+### Indexes
 
 - #29776 Fix #29767, set m_synced = true after Commit()