Doc: B-Tree only has one additional parameter.

Oversight in commit 9f3665fb.

Backpatch: 13-, just like commit 9f3665fb.

doc/src/sgml/ref/create_index.sgml

@@ -425,7 +425,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
    </variablelist>
 
    <para>
-    B-tree indexes also accept these parameters:
+    B-tree indexes additionally accept this parameter:
    </para>
 
    <variablelist>