doc: use more accurate URL for bug reporting

Reported-by: nat@makarevitch.org

Discussion: https://postgr.es/m/172947609746.699.14488791149769110078@wrigleys.postgresql.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/problems.sgml

@@ -304,7 +304,7 @@
   <para>
    Another method is to fill in the bug report web-form available
    at the project's
-   <ulink url="https://www.postgresql.org/">web site</ulink>.
+   <ulink url="https://www.postgresql.org/account/submitbug/">web site</ulink>.
    Entering a bug report this way causes it to be mailed to the
    <email>pgsql-bugs@lists.postgresql.org</email> mailing list.
   </para>