Doc: correct erroneous entry in this week's minor release notes.

The patch to disallow a NULL specification in combination with
GENERATED ... AS IDENTITY applied to both ALWAYS and BY DEFAULT
variants of that clause, not only the former.

Noted by Shay Rojansky.

Discussion: https://postgr.es/m/CADT4RqAwD3A=RvGiQU9AiTK-6VeuXcycwPHmJPv_OBCJFYOEww@mail.gmail.com

Author: tglsfdc

doc/src/sgml/release-12.sgml

@@ -186,7 +186,7 @@ Branch: REL_10_STABLE [e5794cd59] 2021-03-12 11:08:42 -0500
      </para>
 
      <para>
-      <literal>GENERATED ALWAYS AS IDENTITY</literal> implies <literal>NOT
+      <literal>GENERATED ... AS IDENTITY</literal> implies <literal>NOT
       NULL</literal>, so don't allow it to be combined with an
       explicit <literal>NULL</literal> specification.
      </para>