docs: fix duplicate wording in PG 12 release notes

Reported-by: nickb@imap.cc

Discussion: https://postgr.es/m/6b3414e1-fcef-4ad9-b123-b3ab3702d3db@www.fastmail.com

doc/src/sgml/release-12.sgml

@@ -411,7 +411,7 @@ Allow partitions bounds to be any expression (Kyotaro Horiguchi, Tom Lane, Amit
 </para>
 
 <para>
-Expressions are evaluated at table partitioned table creation time.  Previously, only constants were allowed as partitions bounds.
+Expressions are evaluated at partitioned table creation time.  Previously, only constants were allowed as partitions bounds.
 </para>
 </listitem>