minor note in indexing-transactions.md (#2443) (#2444)

Fix inconsistent documents and code

docs/app-dev/indexing-transactions.md

@@ -12,8 +12,8 @@ Let's take a look at the `[tx_index]` config section:
 # What indexer to use for transactions
 #
 # Options:
-#   1) "null" (default)
-#   2) "kv" - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).
+#   1) "null"
+#   2) "kv" (default) - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).
 indexer = "kv"
 
 # Comma-separated list of tags to index (by default the only tag is "tx.hash")