Fix description of pg_class.relispartition in documentation

This missed the fact that indexes can be partitions since 11.

Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20181022031212.GF14282@paquier.xyz

Author: michaelpq

doc/src/sgml/catalogs.sgml

@@ -1948,7 +1948,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <entry><structfield>relispartition</structfield></entry>
       <entry><type>bool</type></entry>
       <entry></entry>
-      <entry>True if table is a partition</entry>
+      <entry>True if table or index is a partition</entry>
      </row>
 
      <row>