Fix typo

Author: alvherre

doc/src/sgml/catalogs.sgml

@@ -2570,7 +2570,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <para>
        <literal>c</literal> = check constraint,
        <literal>f</literal> = foreign key constraint,
-       <literal>f</literal> = not-null constraint (domains only),
+       <literal>n</literal> = not-null constraint (domains only),
        <literal>p</literal> = primary key constraint,
        <literal>u</literal> = unique constraint,
        <literal>t</literal> = constraint trigger,