doc PG 18 relnotes: clarify btree skip-scan item

Reported-by: Peter Geoghegan

Discussion: https://postgr.es/m/CAH2-Wzko57+sT=FcxHHo7jnPLhh35up_5aAvogLtj_D9bATsgQ@mail.gmail.com

Author: bmomjian

doc/src/sgml/release-18.sgml

@@ -469,8 +469,8 @@ Allow skip scans of btree indexes (Peter Geoghegan)
 </para>
 
 <para>
-This allows muti-column btree indexes to be used by queries that only
-reference the second or later indexed columns.
+This allows multi-column btree indexes to be used by queries that only
+equality-reference the second or later indexed columns.
 </para>
 </listitem>