doc: Add missing parenthesis to keycombo

The SIGINT keycombo for the pg_waldump stats emission was lacking a
closing parenthesis.  Backpatch to 15 where this feature was added.

Discussion: https://postgr.es/m/EC39E60E-C8B6-4CDF-8BFA-E4D140446B41@yesql.se
Backpatch-through: 15

doc/src/sgml/ref/pg_waldump.sgml

@@ -263,7 +263,7 @@ PostgreSQL documentation
        <para>
         If <application>pg_waldump</application> is terminated by signal
         <systemitem>SIGINT</systemitem>
-        (<keycombo action="simul"><keycap>Control</keycap><keycap>C</keycap></keycombo>,
+        (<keycombo action="simul"><keycap>Control</keycap><keycap>C</keycap></keycombo>),
         the summary of the statistics computed is displayed up to the
         termination point. This operation is not supported on
         <productname>Windows</productname>.