Improve wording of release notes item

PostgreSQL 11 introduces compress method for SP-GiST opclasses.  That
was mistakenly interpreted as compression support for SP-GiST while
actually that allows lossy representation of leaf keys.

Author: Alexander Korotkov, based on proposal by Darafei Praliaskouski
Discussion: https://postgr.es/m/CAC8Q8tKbYmNdiyWr7hE4GfMY4fbqHKkFziKgrUuWHH6HJQs3og%40mail.gmail.com

doc/src/sgml/release-11.sgml

@@ -933,7 +933,7 @@ same commits as above
 -->
 
        <para>
-        Allow SP-GiST indexes to optionally use compression (Teodor Sigaev,
+        Allow SP-GiST to use lossy representation of leaf keys (Teodor Sigaev,
         Heikki Linnakangas, Alexander Korotkov, Nikita Glukhov)
        </para>