Doc: fix meaning of acronym "btree".

Acronym "btree" better means "multi-way balanced tree" rather than
"multi-way binary tree".

Discussion: https://postgr.es/m/20190105.183532.1686260542006440682.t-ishii%40sraoss.co.jp

doc/src/sgml/btree.sgml

@@ -13,7 +13,7 @@
 
  <para>
   <productname>PostgreSQL</productname> includes an implementation of the
-  standard <acronym>btree</acronym> (multi-way binary tree) index data
+  standard <acronym>btree</acronym> (multi-way balanced tree) index data
   structure.  Any data type that can be sorted into a well-defined linear
   order can be indexed by a btree index.  The only limitation is that an
   index entry cannot exceed approximately one-third of a page (after TOAST